Castle Bleck Key
From the Super Mario Wiki
The Castle Bleck Key is a key found in Super Paper Mario; it is used for opening the door leading to the O'Chunks fight. It is found in a Treasure Chest in Castle Bleck. Soopa Strikers guard the treasure chest of the key. The keys are normally found by flipping in 3D at a staircase. Mimi's key is found if the player hits the red blocks in the room with the spikes one time and the room with fire snakes four times in a row, but the player has to wait for each tremor to finish. These keys also appear for the Dimentio battle and, the Count Bleck fight.