What exactly is this "blood libel" that Sarah Palin spoke of in her response to the Arizona shooting, and what did you think of her speech?

This is from her speech:

”And after the election, we shake hands and get back to work, and often both sides find common ground back in D.C. and elsewhere. If you don’t like a person’s vision for the country, you’re free to debate that vision. If you don’t like their ideas, you’re free to propose better ideas. But, especially within hours of a tragedy unfolding, journalists and pundits should not manufacture a blood libel that serves only to incite the very hatred and violence they purport to condemn. That is reprehensible.”

Can you help me find some of this “blood libel” she spoke of? I mean, I couldn’t find anything about what some pundits were saying, but maybe you can help.

And what did you think about her speech? I thought that it was really planned and a bit deliberate, but it was good… except for that “blood libel” line.