Haden Blackman said on G4 that there will be combat crystals and color crystals. I hope there's a black one.
By the way, you know how all lightsaber blades are white with a colored shroud around it? That's basically how a black saber would look-white with a black shroud.
The combat crystals probably do things such as increasing blade power, so that it inflicts more damage or even shorts out some electrical systems.