Obama would have still won enough electoral votes without the black vote to have won both elections.
You guys realize that the popular vote does not win elections, right?
Also, the black population in the United States is roughly around 13%. Even if we used US age statistics, that would mean 9% of the US population or 28 million people are black and able to vote (over 18). Even if every eligible black voter voted in 2012, they would only make up 20% of the vote. Assuming black voters turned out at roughly the same national average, of 58.8%, that quickly changes to roughly 13% black voter turn out. Roughly 6% of the total vote. Not enough to sway an election, especially given the electoral map.