Block Boo

A Boo Block (originally known as a Boo Buddy Block, also known as a Block Boo ) is a type of Boo capable of transforming into a block called a Ghost Block when threatened.

Super Mario World
Boo Buddy Blocks first appear in Super Mario World. When looked at, a Boo Buddy Block seems to be a normal, gray block but with a unique stone design. When Mario or Luigi takes his eyes off it, however, it slowly begins to transform into a Boo and follow him. When the Boo Buddy Block is in its block form, it can be jumped off. Doing this is required to clear a certain Ghost House, the Choco-Ghost House, as Mario or Luigi must lure several Boo Buddy Blocks into the proper positions so that he can jump off them to reach the high ledge where the exit door awaits.

Super Mario Sunshine
In Super Mario Sunshine, Boo Blocks are known as Pink Boos in English, and they bear a resemblance to Red Boos due to their color. Like normal Boos in this game, they do not care if Mario looks at them, instead turning into Ghost Blocks only if sprayed with F.L.U.D.D.'s water or a Yoshi's juice. Ghost Blocks look very similar to Yoshi Platforms, though their eyes can still be seen on one side. After a lengthy period of time, they revert to their normal form. When not a block, they meanderingly patrol a small area in a similar manner to the Boos in the game, though in a less wide range. They appear solely in Hotel Delfino, specifically around the central pillar in The Hotel Lobby's Secret and over a large pool-like bathtub under an air vent in Mysterious Hotel Delfino.

Yoshi's Story
Boo Blocks return in Yoshi's Story. They appear solely in Stage 6-3: Ghost Castle. Their Ghost Block form is redesigned with pink and white stripes rather than being a solid gray block. In addition, they take a different appearance from normal Boos when a Yoshi is looking away.

Super Mario World: Mario to Yoshi no Bōken Land
In Super Mario World: Mario to Yoshi no Bōken Land, when the Boos turn into Ghost Blocks, they still show their faces, unlike in the original game, where their faces disappear.

Trivia

 * Blocks with Boo faces appear in Mario & Luigi: Bowser's Inside Story in the Energy Hold. However, instead of turning into enemy Boos, these become invisible.