I've never heard of the phrase "tar baby" outside of the Br'er Rabbit story.
No clue how it was (if ever?) applied to blacks and thus became one of an endless list of "offensive" terms regardless of use and context.
For the lurking crazies, I didn't know about Romney's problem with the phrase until I was reading about 'tar baby' at Wikipedia to try to find some kind of racial connotation. They only say it is one but have no background.
I've heard it outside the story by someone referring to a situation hopeless to extract yourself from - but that is a reference to the story.
And exactly how it was used by McCain.