Ebonstone Brick

The whole point of the brick is that it is inert. Raw Ebonstone drags the Corruption around with it: in Hardmode it creeps into whatever it touches, and enough of it in one place counts as the biome itself, which is how people accidentally drive their own NPCs out of a house. Turning it into brick kills both effects. You can put a Corruption themed tower in the middle of a forest and nothing spreads out of it.

It is also the sensible way to store the stuff. A chest of bricks cannot start a biome if it gets placed by mistake, and it can still be cooked back into a look later.

Crimson worlds get the same arrangement through Crimstone Brick, so neither side is short of a safe dark building material.