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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.
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.