Block Boo

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

Super Mario World
Boo Buddy Blocks first appear in Super Mario World. When looked at, it seems to be a normal, gray block, though with a unique stone design. When Mario takes his eyes off it, however, it slowly begins to transform into a Boo and follow him. 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.

Super Mario Sunshine
In Super Mario Sunshine, Boo Blocks are known as Pink Boos in English, and bear a resemblance to Red Boos due to their color. Like normal Boos in this game, they do not care in Mario looks at them, instead only turning into a block if sprayed with F.L.U.D.D.'s water or a Yoshi's juice. The block looks 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 strips rather than being a solid gray block. In addition, they take a different appearance from normal Boos when 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 blocks, they still show their face, 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.