If ever an election was going to produce a riot, it was 2000. And all we had then was the Brooks Brothers Riot.
Look, if Romney were to win thanks to some kind of Willie Horton race-baiting strategy, or by embracing birtherism, then there might be tension. But Romney's not going to do that; he's not the cleverest campaigner ever, but he can see that's a bad idea. If we did have another election decided the way 2000 was, and Romney won as a result, there might be tension. But overall, African-Americans have a lot of experience of watching white guys win elections.
You know there are younger African Americans don't have experience watching black guys lose elections. If the election feels stolen(lost in by a hair in a state that recently passed voter ID) you can expect riots as bad as those in 1960's.