I'm expecting that black metal will just cover the same things as earlier metals have. However it is quite unique from other metals in that it is a really common mob drop.

Much like troll leather and bone piles it is a material that finds you rather than the other way around. Combine that with the fact that fulings can potentially spawn anywhere after a certain surpassing a certain milestone in progression, and the fact that the current endgame isn't actually the endgame. It's going to be impossible to NOT have tons of it.

One thing that I think is very overdue is an improved cart, with more health and reduced speed penalties from heavy loads.