Ancient Blue Brick

There is no recipe and no drop. You carry ordinary blue brick to a shimmer pool and throw it in, and it comes back in the older style. The same trick works on every dungeon brick color, so one trip converts whatever you brought.

Regular dungeon brick is miserable to build with. It shrugs off explosives and refuses to break unless your pickaxe is fully rated for it, which turns any large dungeon themed project into slow manual work. The converted brick drops both restrictions, so an early pickaxe handles it and bombs clear it in bulk.

The trade is that the game no longer reads it as dungeon material. Stacking it up will not create dungeon conditions, so none of the spawns or biome behavior follow it. For a themed build that is exactly the outcome you want. For an enemy farm it is the wrong block.

The texture is the one the dungeon used in very early versions of the game, kept around as a building material rather than restored to the dungeon itself.