From the Super Mario Wiki
Black Shells are shells of unknown origin that appear in the Mario Kart GP series. Its effects are similar to that of a Winged Spiny Shell, though unlike a Winged Spiny Shell, it can only be fired straight and has a higher appearance rate. They explode upon impact of a wall, obstacle, or other racer, sending those caught in the blast soaring for a second. They can also be used as a triple if unlocked.
Other forms of media
One of the artworks from Super Mario Land depicts the Bombshell Koopa with a black shell. In the game, the shell detonates within a few secconds after the hero stomps on the Koopa. Some evidently high-ranking members of the Koopa Troop, such as the Fire Bros. from Super Mario Bros. 3, Black Ninjakoopa, and Lakithunder, are shown to wear non-exploding Black Shells. Additionally, in The Super Mario Bros. Super Show! Koopa Troopas, Lakitus, and Hammer Bros. were portrayed as wearing, as the episode "Mario and Joliet" showed, fire-proof Black Shells.