From the Super Mario Wiki, the Mario encyclopedia
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Black Shells are one of the rare colored variants of the Koopa shell. Unlike the others, this type is capable of exploding. Despite their name, they appear to be a lavender or dark purple instead of black in the Mario Kart Arcade GP sub-series.
Mario Kart Arcade GP 
Black Shells appear as items in Mario Kart Arcade GP. Its effects are similar to that of a Spiny Shell, though unlike a Spiny Shell, it can only be shot forward and straight like a Green Shell, it has a higher appearance rate. They explode upon impact of a wall, obstacle, or other racer, sending those caught in the blast flipping for a second. Triple Black Shells also appear as an unlockable item.
Mario Kart Arcade GP 2
Black Shells reappear with the same function in Mario Kart Arcade GP 2. Triple Black Shells also return as an unlockable item with the same function.
Mario Kart Arcade GP DX
Black Shells once again return with the same function in Mario Kart Arcade GP DX. Triple Black Shells also return as an unlockable item with the same function.