I'm not very smart on the color genetics stuff, but I suppose, in my uneducated opinion, you'd probably end up with a mix that would have some spots, not pure blacks.... (I have mottled blacks and black spanish myself, first season with them....)
Mottle x mottle doesn't produce all pure mottles....kinda like the blue/black/splash color genes work in a way I guess....
lol,
There are a few exceptions, but for the most part, black will dominated the other colors or patterns, especially is it's a black based pattern to start with like the mottled/ They will be split to the mottled, so the F1 babies can then be used to make mottled the next year if bred together or to another mottled.