From the Super Mario Wiki
A Boo Buddy Block is a strange combination of a Boo and a Block only found in Super Mario World and Yoshi's Story. 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 Buddy. They are actually required to clear a certain 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 Buddy Blocks return in Yoshi's Story, renamed as Boo Blocks. 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.
In Super Mario Sunshine, there are Red Boos which turn into solid platforms when squirted. These, too, are needed to complete certain levels.
Blocks that look similar to Boo Buddy Blocks appear in Mario & Luigi: Bowser's Inside Story in the Energy Hold.
- ^ a b Mario Mania, page 51.
- ^ Yoshi's Story official website (Wayback Archive). Retrieved April 22, 2015.