Fun fact: black metal is actually steel that has gone through a form of vulcanisation. Demonic or draconic sulphur are used in the process, to slightly different effect. Demonic vulcanisation gives the metal a matte finish, while draconic is more chromatic.
What you have here is some scraps of black metal. Could be demonic, could be draconic. They're likely off-cuts from the smithing or vulcanisation process, or it's entirely possible that they were bashed off a Black Knight's armour when you boffed them. Regardless, waste-not, want-not. Now you have the material required for some new gear.