You're in luck, because the plains doesn't have mining. Black metal is a loot drop, buuut...

You'll need to go back and get more iron. Black metal is mostly for weapons and tools, and flax and iron is for the armor.

You'd probably want to wear the root chest piece in the plains anyway because it protects much better against deathsquitos and spear throwers. Don't wear it for the boss fight though and watch out for those shamans.