Not every vote for one guy is a vote against the other, except mathmatically. I can vote for John Edwards, for example, without considering Obama for more than five seconds, one way or the other, because (a) in November I'm going to vote for the party's nominee whoever he/she/it is, so if Obama's the nominee I can vote for him then; and (b) Edwards is saying what I want to hear and Obama isn't.
If a black person would've voted for Edwards or Clinton if Obama hadn't run but decided to vote for Obama because he's black, I don't see that as a vote against Edwards and Clinton because they're white. It's a different case for people who only vote when there's a black candidate to vote for.