Crimstone Brick

The raw block spreads Crimson and converts the ground around it into a Crimson biome. The brick does neither, and that is the entire point of it existing.

If you want that dark red look for a base, this is how you get it without your town going hostile, NPCs refusing to move in, or the terrain quietly converting behind your back. It is equally safe for arena floors built near infected ground, where dragging in the raw Crimstone Block would spread the problem you are trying to fight. Worlds with the other evil get Ebonstone Brick for the same job, so the option exists either way.