On GardenStew I have seen Black Holly Hock featured. Recently in a Burpee catalog I saw Black Cat Petunia featured. It has single flowers. The description declared it was an early blooming plant. http://www.pinterest.com/pin/502010689684995641
This link declared Black Petunias "are really good with bright pink and white. Not for a dark corner. Better when they are at eye level." http://www.pinterest.com/pin/228135537350502126