Growing up in sub-Saharan Africa, I was always amazed that Americans of mixed race thought they were black. They sure found out different when they came "back" to Africa - where they were sho'nuff not black.
But I've talked to some of my "black" friends here in the States, and have heard, yeah, we know we have some white ancestry, but since that came from slave-owning rapists, why should we acknowledge it. Made a little sense... but only a little. I have lived in the Southern US now for, oh, 20-some years, and I *still* don't think of American blacks as either black or "african-american" - on the other hand, the schools are so busy doing black this and black that (of course, no "white" stuff, THAT would be racist, yeah, right) that what chance do the kids have of thinking of themselves as people?