Blue Brick

Every world picks a single Dungeon Brick color when it generates and builds the entire Dungeon out of it. Yours is blue, green or pink, and that choice never changes. If a build needs the other two, you either visit another world, open Dungeon Crates and Stockade Crates and hope, run what you have through a Chlorophyte Extractinator, or craft them at a Heavy Assembler while standing in Ecto Mist with Hardmode bars on you.

Dungeon Brick belongs to a very short list of blocks that bombs simply will not remove, and that opens up building options nothing else offers. An arena floored and walled with it survives repeated Dynamite use, which is why it turns up in Wall of Flesh preparation where people blast their own hell bridge. In multiplayer it means another player cannot open your house with a stack of Bombs. Understand the limit though. The brick refuses to break, it does not absorb the blast. Anyone standing behind it and any ordinary block in range still takes the full explosion.

Down in the Dungeon proper you need a Molten Pickaxe or better before you can touch the walls at all. Bricks at or above the surface come up with anything, and so do bricks you place yourself near the middle of the world. That mismatch catches people out. A new player can quarry the surface entrance, carry the bricks down, place them in the underground section to bridge a gap, and then discover the pickaxe that mined them cannot mine them back. Sealing yourself into a Dungeon corridor with your own blocks is an easy mistake and a slow one to undo.

If you are filling out Journey Mode duplication, the Dungeon entrance on the surface holds enough wall tiles on its own to finish the research for the wall version, no descent required.