I saw this in Dave Weigel's Twitter feed:
It's what he calls himself! RT @Shaftan: @DaveWeigel "Dark Horse"?— David Weigel (@daveweigel) January 3, 2011
I wondered what it's about—turns out Weigel was interviewing Herman Cain, who is black and Republican, and wrote "I interviewed the dark horse 2012 GOP presidential candidate..."
Of course, the term "dark horse" goes back to the Polk administration of 1844 when everyone in politics was white—it has no racial connotation. But it goes to show you that you can't say anything without someone thinking it's a gaffe.