AS I UNDERSTAND IT...
Black is a dominant gene that can "cover" other stuff, and depending on what his black breeders are "covering" can determine a lot of how your chicks look.
For instance, when I bred my lavender roo to my black hens I got some fully black chicks, but then I got some "mutts." The mutts suggested that some of the hens were carrying a dominant black gene and appeared black, but were carrying something else under the black. The chicks that hatched fully black suggested that the hens were black covering black.
I hope that makes sense....
The only way I can understand it is to use the paint analogy, which is probably not a good thing since I always say that breeding chickens is not like mixing paint, but.....
A black layer of paint over an already black wall will appear more black, than a black layer of paint over a white wall.... Does that kinda make sense.
And then there is the off chance that I have NO idea what I am talking about, but this is as I understand it....