I'll start with the following: I found the comment wince-worthy, but no big deal. Why? Because it was obvious to me that Palin is utterly clueless about the history of the term and what it means for Jews to hear some uber-WASP lay claim to the phrase to describe what amounts to a scolding. So I was like "blood libel? Really? Yeah, you were really worried about the Cossacks in Wassilia massacring your family last Passover."
What has niggled at me is the idea that somehow Palin was particularly clueless or insensitive. Sadly, she isn't. This sort of nonsense -- people casually tossing around power words from other people's history and culture -- happens with astonishing regularity. Heck, even people who mean well can trigger a crowd when they try to lay claim to something they really have no business laying claim to -- especially when they are ignorant. Who can forget Tommy Thompson's making money is part of the Jewish tradition gaffe? And on the left, Hillary Clinton got glares rather than cheers when she tried to ingratiate herself to an African American audience by referring to Washington DC's lack of voting rights as "the last plantation." (Last Plantation was the the rallying cry of the black community for DC home rule in the 1960s.)
The fact is that context matters. There are lots of possible choices of words and phrases and things to get your idea across without appropriating other people's highly significant words and symbols. I will note that many of Palin's defenders were quite happy to be outraged over the fleeting image of insects crawling on a crucifix in Fire In My Belly. Because, again, symbols and context matter.
There is an old witticism usually attributed to Oscar Wilde: "A gentleman never insults another man -- unintentionally." It's good advice when trying to use symbols or phrases that have strong resonances for other groups not to do so unintentionally.