The Draper screen I have is definitely black. Everyone that sees it in my kitchen automatically assumes it a chalk-board, not a projection screen.
Its as black as any screen material could be.
Im not sure what black diamond screen you are referring to, but if it the SI one, its certainly is not black. I actually have a sample of it sitting on my desk, and its about the color of aluminum foil.
Here is a pic of my Draper RP screen with a sample of SI's Black Diamond 1.4 material.