while we are waiting for Mike's response, i will speculate that the rotors are just coated - for them to be all the way through black, the material must be black, however, material is metal, steel of certain grade, and steel has "metallic" color and i am not aware of "food color" for metals... Cast iron can be black but when you break it, the break line is still with metallic flakes...

now, if the material was not metal, then it could be of a color - do they make rotors from anything other then metal? like the pads, ceramic compound, perhaps? or some kind of NASA space fiber???