According to the U.S census, blacks are 12% of Ohio.
But according to the exit poll, blacks were 15% of voters in Ohio, and they voted 96% for Obama.
Romney lost Ohio by less than 2%, so the incredible get-the-blacks-to-the-polls effort by Democrats was the key.
In contrast, Hispanics were only 3% of Ohio voters, and they voted 42% for Romney, which was a pretty good showing for Romney.
Even in the highly unlikely event that Hispanic voters in Ohio voted for Romney at the same percentage as white voters in Ohio (which is 58%), Romney would still have lost Ohio.
To win Ohio, Romney needed to either suppress the black vote, or get more whites to vote for him.