There are too many people in this country uncomfortable with the idea of anyone other than a white male leader that I don't think anything will happen for a while. I'm 21, and I think I MIGHT see it happen. Large parts of this country are still as rascist and sexist as they think they can get away with.
But when it does happen, I think it will be a black, male, Democrat. I hate to stereotype, but Democrat voters are concentrated in larger cities, and cities are generally more progressive when it comes to that kind of thing. As to why black before female? Look at the workplace for clues. People are, on average, more comfortable with a black manager than a female one. At least that seems to be the case.