Andrew Brietbart used the phrase in a tweet last night (though it's likely Palin's video, posted in the early morning hours Wednesday, was filmed before that tweet).
"Blood libel" popped up in a Wall Street Journal op-ed by professor and Pajamas TV host Glenn Reynolds. "Where is the decency in blood libel?" he wrote on Monday, Jan. 10.
Adam Graham of Renew America was the first to use the phrase in reference to the Tucson shootings, according to a search of Google News.
But the very first reference to "blood libel" in the wake of the Tucson massacre came from political consultant Jon Henke, who used the phrase on Twitter the same day as the shootings, according to Google's realtime search.
Reaction to Palin's use of the phrase, which refers to the Middle Age anti-Semitic accusation that Jews killed Christian children and used their blood in ritual food for Passover, was -- like all things Palin -- mixed.
A Jewish Democratic political consultant told Politico it was "absolutely inappropriate."
"The blood libel is something anti-Semites have historically used in Europe as an excuse to murder Jews -- the comparison is stupid. Jews and rational people will find it objectionable," said Hank Sheinkopf. "This will forever link her to the events in Tucson. It deepens the hole she's already dug for herself."
But Reynolds defended his -- and her -- use of the phrase in an e-mail to Ben Smith. "I am of course aware -- and I imagine the very pro-Israel Palin is, too -- of *The* Blood Libel from medieval times, but one sees false associations with murder called *a* blood libel without reference to that," Reynolds wrote.
The National Review also says the phrase is more common than people think.
Given that Palin is a strong supporter of Israel, it's likely she knew the history of the phrase. Even a quick Google search would have turned up this Wikipedia entry as the first result.