The scales of a black dragon are sturdy things, capable of deflecting all but the strongest attacks. It’s not so much that they’re harder than other scales, but that the creature that bore them is so determined to survive that that tenacity passes on to one who wears it.
There are no stories about people making bonds of friendship with the black dragons of Ashenfall, because the dragons do not allow anyone to think of them as their equals. They are the masters, the superior species and the garou and goblins of the Scorned Wilderness have long been taught to know their place.
There are certainly plenty who would challenge them, true, but over the centuries the list of successful challengers has yet to pass into double digits.