Gather 4 gravel, 4 sand, and 1 black dye
Place the black dye in the center of the crafting table
Place the 4 gravel in the top-left, top-right, bottom-left, and bottom-right slots
Place the 4 sand in the remaining four slots
Craft black concrete powder
Place the black concrete powder on the ground
Pour water on the powder or let it touch water
Collect the resulting black concrete with a pickaxe