Victim-blaming occurs when a victim is held partially or wholly responsible for a crime or abuse against them. (Typical examples include blaming the creator of a cartoon criticising Islam for the violent reaction of Muslims and blaming a rape victim for encouraging her own rape.) Much has been said about whether this is or is not fallacious in general, but I think it's also important to make a careful consideration of different types of responsibility when determining whether somebody is blaming a victim in a specific case. For instance, I've often seen the mention of mitigating factors in legal contexts criticized as victim-blaming. I think that is a mistake.
Types of Responsibility
The word "responsibility" has different meanings in different contexts, and general speech tends to lump them all together. However, for consideration of blame, it is important to consider the use of the word in its context, as different contexts have different meanings of responsibility and different rules for how responsibility is assigned. In thinking about this, I've identified four types of responsibility. There may be more.
Consider the following hypothetical situation, which will be used as an example throughout: A young white man decides to be an asshole, do a foolish prank at the urging of his friends, and/or make a point about the right to free speech by putting on a shirt that says "All niggers should hang" and walking through a black ghetto. Unsurprisingly, a black youth takes offense and kills him. Does the white guy share any responsibility for his murder, or is it completely the black guy's fault?
Because victim-blaming is most often invoked in the context of criminal acts, it's common to hear discussions of legal responsibility. In the example, the white guy has no legal responsibility for his murder. That is true in every case, even when there are clear grounds for a counter-suit or a related suit against the plaintiff. (The counter-suit would be a separate legal question with a separate assignment of responsibility.)
I don't think anybody blames victims legally, but I've often heard the discussion of mitigating factors in a legal context criticized as victim-blaming. While it's true that the behavior of the victim can be a mitigating factor in reducing the legal responsibility of the perpetrator, this does not assign any legal responsibility to the victim. As an example, if a man comes home to find his wife and another man having sex, and he kills the man in a rage, a court may find that the victim's behavior was so shocking as to incite a crime of passion, and thereby find that the husband's responsibility for the murder is reduced or even eliminated. That's not to say that the victim is at all legally responsible for his own murder! Legal responsibility is not a zero-sum game wherein if the perpetrator's responsibility is reduced, the victim's responsibility must be increased. (It's even possible for an illegal act to be performed such that nobody is legally responsible.) So the discussion of mitigating factors in a legal context is not in itself victim-blaming.
Moral responsibility deals with the question of whether the victim "deserved it". Did the white guy in this example in any way deserve to be killed? What if he was merely beat up? Would he have deserved it then? Does a convicted child rapist who has served his sentence deserve poor treatment by the public after his release? Because there is no objective morality, we can't expect to find definitive answers to these questions, and there are enough schools of moral thought that you'll likely get wide ranges of answers in surveys of both the public and academia. Nonetheless, any assignment of moral responsibility to the victim is most assuredly victim-blaming.
However, it is important to remember that a statement morally blaming the victim is not an objective statement about the victim. Rather, it's a subjective statement that should be viewed in light of the moral framework in which it was uttered. Forgetting the framework and treating it as an objective statement leads to conversations like "The victim is partly responsible in this case" followed by "Victims are never responsible!" Like a Christian and a Muslim arguing that Jesus "was too" or "was not" divine because their respective holy books say so, they can go round and round forever without making any progress toward understanding or resolution of the question. (I've also seen someone infer from a statement that the perpetrator is less morally responsible that the victim is therefore more responsible, and get upset. It's possible that the two people simply make different assumptions about the degree to which moral responsibility is a zero-sum game.)
It's entirely possible that as far as moral responsibility goes, the victim, his defenders, his critics, and the perpetrator could all have different moral frameworks by which they assign blame, and in the contexts of their frameworks it's possible that they could all be "right". In an argument about the assignment of moral responsibility, then, it is more illuminating to explicitly consider the moral framework of the person you're talking with and critique the framework itself. While there are no objectively correct moral frameworks, that's not to say they are all equally good. Most moral frameworks are flawed by internal inconsistencies, counterproductive ideas, and other logical problems. By finding these problems and pointing them out, you're far more likely to advance the conversation usefully than if you just restate a premise of your framework as though it was an objective fact.
A third type of responsibility is what I'll call "rational responsibility". I call it "rational responsibility" because it's a statement about the reasoning of the victim, who is assumed to be a more-or-less rational agent. I'll give two extreme examples that I hope should make the general concept clear. A prisoner of war makes a suicidal diversion to draw the guards' attention away from the other prisoners so they can escape. A small child chases a ball into the street despite a clearly oncoming car and gets killed. In the first example, when the POW decided to make his diversion, he was sure he would be killed. In the second case, despite it being obvious to onlookers that the child would be killed, the possibility never entered into the child's mind when he decided to run into the street. Although the actions of both people caused easily predictable deaths, I would say that the POW bears rational responsibility for his death while the child does not. The POW knew that making the diversion would entail his killing but still chose to do it; therefore he chose to be killed. The child did not know what it was doing, and so cannot be said to have chosen death.
More specifically, a person's degree of rational responsibility r can be taken as the expected probability p of an outcome of an action in the mind of the person taking the action, discounted by the degree of coercion c (where 0 is a free choice and 1 is a forced choice): r = p(1-c). Note that the expected probability does not have to bear any relation to the actual probability. If a person expects a 50% chance of outcome X if they do action A, then they are 50% rationally responsible if they freely choose to do A and X occurs, even if the actual probability was 100% or 0.01%. Similarly, if a person does not expect that X can occur, they have no rational responsibility even if it does. The possibility does not have to enter into conscious deliberation either. A person's unconscious probabilistic reasoning suffices to make them rationally responsible.
Rational responsibility, then, is essentially a measure of how much a person chose* a consequence of their actions. But because it is concerned only with the victim's personal expectation of what might have happened, onlookers can only infer this responsibility. I believe it is the logic behind statements such as "he should have known!" — inferring that he must have expected that X was likely, and so bears some responsibility for choosing an action that (predictably) lead to X.
In our example, say the white guy's friend said "Are you crazy? You're gonna get killed!" and he replied "Nah, probably not," assuming the chance was only about 10%. Then he would be 10% rationally responsible for his murder. Is this victim-blaming? Perhaps. As defined, rational responsibility is orthogonal to moral responsibility (but may be related to the reasoning used to determine legal responsibility). High rational responsibility for a death does not necessarily imply any moral responsibility for it, or any foolishness or wisdom in taking the action that led to it. Nor does a lack of rational responsibility necessarily imply a lack of moral (or legal) responsibility. So I would tend to say that a determination of rational responsibility, narrowly considered, is not victim-blaming. Nonetheless, statements such as "he should have known!" have an air of moral judgment about them — that knowing a particular outcome of an action was significantly possible and doing it anyway implies some moral responsibility for the outcome. However, like other moral judgments, it flows from a particular moral framework. No doubt many people have such a rule in their moral frameworks, and when they use the language of rational responsibility to make an implied statement about moral responsibility, then I would say they are engaging in victim-blaming.
The final type of responsibility I'll consider here is fairly trivial, but I've occasionally seen people confuse it for other types of responsibility. "Causal responsibility" is the term I use for taking an action that contributes to the causal chain of events that leads to a particular outcome. Almost everybody has some causal responsibility for what happens to them. If Mr. Nail is walking down the sidewalk and a construction worker accidentally drops a hammer on his head, they both have some causal responsibility. The construction worker dropped the hammer and Nail walked down the wrong street at the wrong time. (And somebody hired that construction worker to be there in the first place.) Having causal responsibility implies nothing about legal, moral, or rational responsibility. It is simply a statement of fact about who did what. As such, it is not victim-blaming, but if somebody was to use the language of causal responsibility to make a veiled moral judgment, then that would be victim-blaming.
There are several ways that a person can be responsible for an outcome, and despite being orthogonal to each other they are often conflated. While most uses of the word don't require consideration of different types of responsibility, accusations of victim-blaming in response to an assignment of responsibility do. On one hand, only assignments of moral responsibility can be properly called victim-blaming, and on the other, one should be sensitive to moral judgments disguised in the language of other types of responsibility.
I have avoided passing judgment on whether victim-blaming is always or sometimes right or wrong, mostly because it's a big topic that's irrelevant to my main point, which is that people on both sides of the question should be more careful about what they're saying since they tend to talk past each other most of the time.
* I realize that free will is almost certainly an illusion and so people cannot make truly free choices, but I believe that words like "choice" and "coercion" can be redefined in light of this so we can keep using normal language instead of having to reword everything in the language of information processing. :-)