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A Boo Block (originally known as Boo Buddy Block) is a combination of a Boo and a Block first appearing in Super Mario World. When looked at, it seems to be a normal, gray block. But if Mario takes his eyes off it, it slowly begins to transform into a Boo. They are actually required to clear a certain Ghost House, Choco-Ghost House, as Mario must lure them towards him, turn around, and jump on and leap off them to reach a high ledge, where the exit door awaits. In Super Mario World: Mario to Yoshi no Bōken Land, when the Boos turn into blocks, they still show their face, but in the original game, their faces disappear.
Boo Blocks return in Yoshi's Story, now also known as Block Boos. They appear solely in Stage 6-3: Ghost Castle. Their Ghost Block form is redesigned with pink and white strips rather than being a solid gray block.