Blue Shells are blue-colored Koopa Troopa shells. Blue Shells can, as shown in New Super Mario Bros., be worn by humans such as Mario, turning him into Shell Mario and giving the ability to slide as a Koopa Shell to places and bump on Brick Blocks and break them. They, however, first appeared in Super Mario World, and were kicked by Blue Koopa Troopas.
The Blue Shell is not to be confused by the Blue Spiny Shell, or Blue Flying Spiny Shell, an Blue Shell similar to Spiny Shells (with wings attached in recent games) that attacked the leading opponent in Mario Kart series.
Super Mario Galaxy Odyssey
Blue Shells appear, along with Green, Red, and Gold Koopa Shells in the game Super Mario Galaxy Odyssey. Unlike other shells, which are thrown when Mario spins, Mario instead goes inside of the shell and slides around, breaking down obstacles and enemies, and whenever he hits a wall he comes out of the shell and holds it in his hand again. This ability is virtually the same as the Rock Mario powerup from Galaxy 2 and the shell abilitity from Super Mario 3D World, and is considered a replacement for both. Because this shell is often required to pass a puzzle, it will sometimes be seen in Item Crystals. Additionally, underwater they move Mario slower than the other shells but allow him to be underwater indefinitely, without getting more air. Blue Shelled Koopas are much rarer than other types and respawn if killed.