From the Super Mario Wiki
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Black Rocks are obstacles in Mario & Luigi: Superstar Saga. They are found throughout the Beanbean Kingdom, serving as roadblocks for Mario and Luigi. They can be destroyed only by Ultra Hammers, which the Hammerhead Bros. forge for Mario and Luigi at their home in the Beanbean Castle outskirts. Several Black Rocks obstruct the entrance to Joke's End, requiring these Ultra Hammers if the brothers proceed.
In Mario & Luigi: Superstar Saga + Bowser's Minions, they appear as light brown-colored rocks instead.