Typically, shading on a black subject would be negative, meaning the shading is actually lighter than the base color, not darker. Shading is done to replicate grime in most cases or actual shaded light in others. Grime and dirt is not black so shows up lighter on a black subject. Since I suspect you are replicating grime I'd go with a lightened base coat, a very very dark gray. Almost black. Then apply your shading with a slightly lighter version of your base coat like you normally would. I'm about to attempt the very same thing on an aircraft.