Batman: The Brave and the Bold – Dark Costume Cheat: Doesn't Exist

There is no cheat code to unlock a "Dark Costume" in Batman: The Brave and the Bold (Wii/DS). That costume does not exist in the game.

Available costumes include the classic blue/grey, The Batman animated series suit, and Batman Beyond suit. You may be thinking of a different game (e.g., Arkham series) or a different outfit.