I wondered this too. I think of Obama as black, and one day thought to myself “but why, if he’s also half-white”? I thought about it for a while, and here’s what I came up with:
First, he “looks” black to me; had I not been told, I wouldn’t have even thought to wonder if he had one white parent. So I think it’s safe to assume that he is now, and has been in the past, perceived by most Americans as black (whether that’s really correct and enlightened, or not). So symbolically, regardless of his actual genetic make-up, he’s “black” to most Americans, and will certainly be called our first black president if he’s elected.
Secondly, he married a black woman. That suggests (though it doesn’t prove, of course) that he self-identifies as black. I mean, it’s possible that he doesn’t think of himself as black, but rather as someone in an interracial/mixed marriage.. but I tend to think it means he thinks of himself as black (or more black than white, or something along those lines).
In any case, I have a tremendous amount of respect for the man, and I’d vote for him no matter what color his skin was.