There apparently was a black lightsaber in The Force Unleashed video game, but it was some unlockable thing or some such and it wasn't really considered canon. Then it showed up in The Clone Wars and was canonized.
To be honest, I'm not really fond of the concept or how it was realized. The wonkiness of the physics behind "black light" aside, the thing looks like a regular sword that someone recoloured into an energy blade at the last minute.