So, I got this from a bit of this (and lovely story).

In domestic dog breeds, black is far more prevalent than gray. Schnauzers are a gray breed. But we have black labs, black and white border collies, etc.

Could this be because we noticed that black wolves didn't get sick as often and thus selected for it. Meanwhile, the pup survival advantage of gray coats would not be as much of a factor with humans helping protect the pups.

It's just a thought...distemper is nasty and not something you want going through your kennel.