From the Super Mario Wiki
“When they hit something, they go BOOM!”
—Purple Toad, Super Mario Galaxy
A Bomb Boo is an explosive, black-colored variety of Boo. They are found only in Super Mario Galaxy and its sequel, Super Mario Galaxy 2. Bomb Boos have so far only made appearances in two galaxies: the Ghostly Galaxy; during the missions "Beware of Bouldergeist" and "Bouldergeist's Daredevil Run" and in the Boss Blitz Galaxy; during the missions "Throwback Throwdown" and "Throwback Throwdown Speed Run". Bomb Boos float lazily back and forth, exploding after some time, but they do move towards Mario or Luigi if they get close to them. If the player Spins near one, Mario or Luigi will grab its tongue, and it will get spun around until it hits something. If Mario or Luigi holds on to the Bomb Boo for too long, they will let go of it.
swinging six Bomb Boos
around at once.
If the player holds it for too long, it will begin to flash red. If the player does not let go of it then, the Bomb Boo will explode, injuring the player (though by continuing to spin, the Bomb Boo can be swung wider and faster until it flies out of Mario's grip or hits something). Mario or Luigi can also hold multiple Bomb Boos at the same time, specifically six is the maximum that can be encountered and grasped.
In Super Mario Galaxy 2, Bouldergeist re-appears, along with four other Super Mario Galaxy bosses, in the Boss Blitz Galaxy. Because of Bouldergeist's re-appearance, Bomb Boos make a comeback (so Mario can damage the boss).
Names in other languages
||Bomb Boo Hoo (Boo Hoo being the German name of Boo)
||shy explosive ghost