The black vote passed that of whites (relatively speaking) for the first time in history during the 2012 election.
http://www.dailymail.co.uk/news/article-2316115/The-black-vote-passed-white-vote-time-history-2012-election.html
The key fact here is that this didn’t happen in 2008, when it could be explained as an enthusiastic reaction to the first black presidential candidate.
It happened in 2012, in spite of efforts to suppress minority votes. The demographic time bomb may have a shorter fuse than we thought, if you don’t mind me mixing my metaphors.