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. It is possible that these are the shells of a Bombshell Koopa, as they are both black and they both explode. They can also be used as a triple if unlocked.
Some evidently high-ranking members of the Koopa Troop, such as some Fire Bros., 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 Brothers were portrayed as wearing, as the episode "Mario and Joliet" showed, fire-proof Black Shells.