I've been hearing all day how "blood libel" is some sort of Jewish thing, that it's anti-semitic. This is news to me, because I've also heard the term used to describe attacks on early Christians ("blood" being a reference to Holy Communion).
As I understand it, it's any situation where one group falsely accuses another group of heinous acts, then uses those phony accusations as justification to abuse them in some way. It need not involve religion at all.
"Joe feeds little children to sharks, therefore we should go steal his car."
As I understand the term, it applies perfectly to the Palin situation. The left accuses Palin of being complicit in the murders, then uses that as justification to savage her politically.