From the Super Mario Wiki
Elite Goombules are a stronger variation of Goombule fought only once inside Bowser's body in Mario & Luigi: Bowser's Inside Story. They are encountered in the Trash Pit. They have the exact same moves as regular Goombules, and like their weaker counterpart, Elite Goombules have an egg yolk-shaped body with a yellow core in the center, but are blue instead of pink and wear Buzzy Beetle Shells as helmets that deflect jump attacks and counterattacks. To defeat one, the bros. have to use their newly learned Green Shell Special Attack obtained by collecting all ten Attack Pieces from the Trash Pit.
In Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, their helmets are dark blue rather than black.
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