Is 'Blood libel' an established expression?

I came across the word, 'Blood libel,' apparently associated with Ms. Sarah Palin's agressive statement which was zeroed in at Democratic lowmakers in today's Washington Post.
Neither Concise Oxford Dictionary nor Oxford American Dictionary carries this word. The latter defines 'libel' as:

  1. Published falese statement that damages a person's reputation.
  2. A statement or anything that brings discredit on a person or thing.

Both of which doesn't seem to me fitting to 'Blood libel' used in the context of the above statements. I think I can guess what the word means here, but is 'Blood libel' a broadly received expression in U.S. and U.K? Who coined this word on the earth?
I'm asking this because it's in many cases shown in parentheses.

Quotes:

Questions about whether Palin knew what she was getting into by using "blood libel" miss the mark. Right now, she is as much provocateur as politician.

And:

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.


Solution 1:

Blood libels seems to be an established expression, one used without quotes (") nor parenthesis.

It is employed as such in this article "Sarah Palin causes a controversy with the phrase “blood libel.” What does it mean, and why are people upset?".
Benjol also mentions the BBC article "What does 'blood libel' mean?".

The Wikipedia article details its long history.