Why can't you print with light gray?
Inks typically work in a subtractive manner rather than an additive one. That is, an ink simply acts as a filter - absorbing wavelengths outside of the desired colour but depending on the underlying paper to actually reflect the remaining light. Light grey ink isn't a mixture of white and black, it's simply a black ink with a lower density so it absorbs less light passing through it (ie effectively a semi-transparent black).
A black paper will absorb most of the light hitting it's surface, so there isn't enough light passing through the ink for you to see any descernable colour. In order to print on a black paper, the printer would need to be able to lay down a white (ie reflective) undercoat on any areas that weren't black so that enough light was reflected for the rest of the inks to function.