Yes many people have used the phrase "blood libel" - including yours truly - but I nodded to the term's history when I wrote:
Paladino speaks of “perverts who target our children and seek to destroy their lives.” This is the gay equivalent of the medieval (and Islamist) blood-libel against Jews.
Not many people use the anti-Semitic meme of Jews sucking others blood about a Jewish financier - but Fox News put it right up there, thanks to Glenn Beck.
Adam Serwer contends that blood libel "is not wrongfully assigning guilt to an individual for murder, but rather assigning guilt collectively to an entire group of people and then using it to justify violence against them":
Jim Geraghty has a list of examples of other people using the term in a political context, but some of them are actually appropriate, others less so. Eugene Robinson's reference to the Reconstruction Era lie that black men went around raping white women as a form of blood libel fits the above description, Andrew Sullivan's use of the term to describe anti-gay-rights politicians accusing gays of all being child molesters is similarly appropriate. It's about using a falsehood to establish collective guilt in order to justify collective punishment, not mean things said about an individual person.
The phrase used by Palin, if a little off, as Adam notes, does not offend me. The timing of it left my jaw on the carpet. I still wonder who wrote it.