The ? Block is an item that falls under the category of "Sports" in the Supplies. It is a block with a question mark on it, being a cameo of the ? Blocks from Super Mario Bros. The Terry Cloth Cube and Dice Cushion can be held in the mouth, but the ? Block can only be pushed around. It is the biggest block in the series.
DS: "Even if you hit it, nothing pops out. Too bad!"
The description is a reference to hitting a ? Block to obtain coins or items in Super Mario Bros.