[Chris Wild]

Every black dog we've owned has had a black undercoat. We had a bi-color who also had a black undercoat. And have had dark black & tans (blanket pattern) with black undercoats under their black markings and others with dark gray undercoats. Our sables have all had light gray, beige or carmel colored undercoats.

So no, I don't think black undercoats are restricted to only black dogs. Black dogs will always have black undercoats, but other dark dogs like Bis and melanistic black & tans can have them too.