There are black metal bands out there.
Sepultra has a black lead singer from Brazil -
http://sepultura.uol.com.br/v7/
Straight Line Stitch with Alexis Brown -
http://www.myspace.com/straightlinestitch
There's more.
In the past, from my experience with record labels, they wouldn't promote black rock or metal. Black people were discouraged from even trying. I went through this, even to the point of getting shunned by both A&R on down to other white rock bands. It was okay, if you would be that inconspicuous member in the background, but not up front.
However, the blame can't all fall on white people for this. Black people have contributed just as much to the problem by failing to support black rock as well, talking smack about Jimi Hendrix. When I was young, I remember Hendrix being called a sell-out. I got into a huge row on another black-owned board about my participation in rock and my marriage to a non-black man, so unless black people get with the program and support it, you won't find much, and until then, black rock acts need to stop expecting them to.
Chaeya