On this page of the Valheim game guide, you will learn how to obtain Black metal scraps and what you can use them for.
Black metal scraps are used in the Blast furnace to smelt Black metal bars. You can use this valuable resource to make black metal weapons of very good quality, such as atgeir, axe and knife.
The Black metal scrap can be obtained after defeating the Fulings - small green creatures similar to goblins. The Fulings live in large camps in the Plains biome and are extremely powerful.
After you defeat the Yagluth boss, the Fulings will start appearing in other biomes (Meadows, Black Forest), but only at night.