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Blah!
A Boo Blah is a kind of ghost that appears in Super Mario World 2: Yoshi's Island. This creature sticks to the ceiling or the floor, and jump up periodically. A large majority live inside Bigger Boo's fort along with other spirits and Boos. They are invincible, but are not a problem, they only hurt if you somehow bump into them when they jump up.
They could have been the inspiration for Luigi's Mansion's Ceiling Surprises, which are similar to them.