black metal is used in a number of ammo recipes, including the ballista, as well as being used for the best chests (6 at a time I think). It would be a big bonus if they would make a black metal rebar/iron-bar type thing for home construction. That would make the game much, much kinder. But the answer is simply effort/reward. Black metal all but falls on your head from the sky in quantities so high you start tossing it aside. It takes 0 effort to get, so it isn't the most useful material.