"Blood libel", had until fairly recently, only ever been used as a term to describe the unpleasant myth that Jewish people killed Christian children as part of some bizarre ritual. It's one of many racist slurs that Jewish people have had to put up with during an extended period of persecution. So why did Palin choose to use such an esoteric expression to respond to her accusers?
This article offers some suggestions and makes some fascinating points about how language changes as a result of social networking technologies and the web.
The first use of the phrase I uncovered came on January 9, one day after the shooting, on the website Renew America. As conservative activist Adam Graham put it: “When someone on the left says that the Tea Party movement is responsible for the shooting in Tucson, they are leveling the political equivalent of a blood libel that blames an entire political movement for the actions of a person who in all likelihood had no connection to the movement.” Note that Graham links to the Wikipedia page on “blood libel,” demonstrating knowledge of the traditional meaning of the term.
They also link to this article which makes the intriguing suggestion that internet "memes" spread more like a heartbeat than a virus, a model for language change that might start to rival the traditional wave and S-curve models.