[troubletb]

It is called anal glands and if she doesn't know what she is donig she can hurt the poor boy..( and besides it smells really bad too!)
It could also be dried stool too, Give him a bath or just put his butt in some warm water and make him sit for a bit. Does it come off? Could he have tapeworms? Either, anal glands need to be expressed or he could have tapeworms (reason to drag his butt on the floor) black crusty stuf??? I would say without seeing it , its probably dried stool.
Good Luck, let us know