Florida, 0.88% Black population 16%, if as you state the black population split 50-50, Florida goes to Romney
North Carolina, 2.04% Black population 26% but irrelevant because Romney won the state.
Ohio, 2.98% Black population 12% if as you state the black population split 50-50, Ohio goes to Romney
Virginia, 3.87% Black population 20%, if as you state the black population split 50-50, Virginia goes to Romney
That cuts the electoral vote from 332-306 to 272-266 for Obama. Pennsylvania has a black population of 11% and Obama won it by 5.39%, talk about a tight race if we used the 50-50 deal. There are a couple of other states that would have been pretty close. But the 2012 election would have revolved around Pennsylvania.
I love messing with this stuff. With 25% of the black vote, Romney wins the popular vote and those 3 states I mentioned above. He would not have needed 50% to carry Virginia, Florida and Ohio. 25% would have done the trick. As for the white vote in 2012 Romney won that 59-39, in 2008 McCain won it 55-43. Democratic Candidates with the largest share of the white vote are 1. Carter 48%, 2. Clinton 44% Obama 43%.