I think it's because with black fur they have black skin. Like when you have an animal with light fur and dark spots if you shave the dark part there's a dark spot where the dark fur was. Animals with light fur will have pink noses because the reast of their skin is light. That's juast a guess though. I don't know if you shaved a black cat if all of it's skin would be black like it's nose.
Not true. My cat momo is all black except for his nose and the little soft thingys on the bottom of his feet, which are pink.