You're right, if he was up for the big job you probably would see a surge in black voters but it'd still be only within that 13%. I think the reality is that his color will be a liability in a presidential race.
I don't think to the degree that you're asserting. I mean, is ANYONE who'd otherwise vote Democrat NOT going to vote for Obama, if he wins the nomination, because he's not white? Even John Kerry (who was a crappy candidate, however good a President he might have been) almost beat GWB in the middle of a war, and one of the things that the Iowa caucuses were interestingly indicative of is the fact that a lot of "Independents" and even a few Republicans felt that Barack was worth their time.
I do agree with Sarah's assessment of how this breaks down along sexual/racial lines. That said, another question that arises is, is anyone who'd otherwise vote Democrat NOT going to vote for Hillary, if she wins the nomination, because she's a woman? I actually find it easier to believe that, than that they'd not vote for a black man (both suggestions are disgustingly and alternately sad and infuriating).