Such an intelligent question deserves a good answer - I’ll try my best.
My understanding is that yes, blacks voted close to 90% Democrat in 2000 and 2004, so there may not be much more of the black vote to go to Obama. But he might increase turnout, which is less predictable. So I would say that Obama has a poteltial reserve of “new” voters, of undetermined size.
potelial = potential
McCain's reserve MAY be the Hispanic vote. But what he MAY pick up in Hispanic votes he will likely lose in the hard conservative vote. I don't care, as long as its enough to beat the Effete Obama. JMO.