I'm thinking of writing more on this, but for now, consider the black petunia. Called 'Black Velvet,' it came on the market in 2011 to generally favorable reviews. Growers loved it, but there was some hesitation, too. Black - far as I know - has never been a popular color in the garden, and some folks hesitated. But this petunia really is cool.
I found it at Terrain at Styers in Chadds Ford the other day, along with 'Phantom,' another black petunia with a cream/yellow star in the center. There's at least one other black petunia - 'Pinstripe' - and a few other darkly dark flowers out there, among them hollyhocks and pansies.
Do these plants fade into the background? Get lost in the sun or mismatched too often with the wrong partners or venues? No doubt, black is tricky, but the online gurus suggest white and chartreuse flowers or foliage as good partners. (Happy to see they do not advise red.)