The black color is an indicator of the chakra that the stone will work on. It is not so much the color of the stone, but its energetical properties. In general, black stones are grounding and protection.

I hope that helps.

I was doing some research and found this on black stones:

Black is the color of regeneration, black represents beginnings after natural endings in life. Black banishes negativity, helps to heal old sorrows and allows you to move on from redundant or destructive relationships.

Black slows down an overactive, stressed system. Healis the skeletal system, especially the spine, removes blockages in the lower body and overcomes trauma and shock. It is usesful in dibilitating illness.

From Cassandra Eassom's Illustrated Directory of Healing Crystals.

I hope that helps.