Lots of folks run for Senator as their first office and win, President is a different matter. And now that Hillary has that all important first win it's a non-issue for her. I don't think Condi would take a significant portion of the black vote, problem with the black voting block is that they believe strongly in the "uncle tom" concept, Condi has already been quite effectively given that brand, which makes her not black as far as they're concerned, and therefore she offers no inspiration to switch parties.
She could change that "uncle tom" attitude, and besides, Cosby and conservative black FReepers have already shown that it's on its way out bigtime. I often look to black Americans for moral clarity these days.
Pealing away as little as an additional 1-2% of the black vote would cause major problems for the Democrats.