From the Super Mario Wiki, the Mario encyclopedia
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Their most notable appearance is as an item in Mario Super Sluggers. These Boos make the ball invisible so the player in the outfield cannot see it. The Mini Boos are found in Wario City and are required for King Boo to join the player's team in Challenge Mode. A fielder is still able to make the catch if they go to the area the ball would be in.
Boos have been shown to show the unusual trait of being able to combine and create a bigger Boo. This is shown in Boolossus from Luigi's Mansion and Boo's offensive Power shot in Mario Power Tennis. Boolossus is a large Boo that is created by fifteen Boos combining together. Boo's power shot shows him summoning many Boos into the form of a large Boo-like tennis racket, hitting the ball with it, and then allowing the Boos to disperse and help cloak the ball in hopes of earning a point. However, just as fielders are able to still make the catch despite the Mini Boos hiding the ball, opposing players can still hit the tennis ball successfully if they know where to be.
Similar small, transparent Boos can be seen in Super Mario Galaxy on the finish line planet of the Ghostly Galaxy during the mission A Very Spooky Sprint. They merely come from the planet's surface a short distance before disappearing.