
Sure, there are general rules for coat colour, but of course there are always going to be exceptions in some breeds. I didn’t know black was recessive in GSDs, but that does make sense in a strange kind of way!
The “default” or wild colour of mammals is usually agouti, which is where the hair shaft is black with one or several red stripes giving a sort of brown ticked effect (a wild rabbit is a great example). Solid colours are recessive to agouti. So maybe GSDs work in a similar way, with solid black colour recessive to tan+black or sable?
In many animals having solid red and black at the same time is recessive to just solid black.