Bomb Boos are black Boos that will explode on contact. They first appeared in Super Mario Galaxy for the Wii in the Ghostly Galaxy, where Mario could swing the boos around using their tongue. If the hit something, then they would explode, possibly leading Mario to a previously inaccessible area. They're also used against Bouldergeist, the galaxy's boss.
This Mario-related article is a stub. Please help by expanding it.