BWM should be black with no white hairs at all anywhere. Some don't get any ever, not even in old age while others grey quite a bit. If you want to breed to a high standard you should not breed from a Ram who has white on his face. Grey in the britches after the age of two say is ok but not ideal but def no white on face, ever!
I used to breed and show them before moving abroad. They are my fav breed, wish I could get them here!