Obsidian Brick

The ruined houses scattered across the Underworld are already built out of this, and any pickaxe will take it down. A single trip through hell with a decent mining loadout gives you more of the block than crafting would, and you were going down there for hellstone anyway. Crafting is for topping up a build, not for the first stack.

It is flat and dark, which makes it read as stone rather than as decoration, so it suits hell builds and dungeon interiors and gets lost anywhere bright. Look closely at one tile and the texture has a face in it, which is easy to miss until someone points it out and then hard to unsee.