No, not a character from his books, but a character from, as mentioned, a Minstrel show. Well, actually, it appears that it was a "Mr Bones" in the show and King has changed it to Mrs Bones beause he is referring to a woman. Or there could have been a Mrs Bones, I don't know.
Anyway, he uses this simile because the show consisted of white people who would wear make-up to look like black people, hence using it to describe the woman who is covered in chocolate.
Here's more information about the minstrel show, anyway:
Minstrel show - Wikipedia, the free encyclopedia
(Ha, Pyan, you nutter
)