Blue Shells are Koopa Shells featuring a deep blue carapace. They first appeared in Super Mario World, causing their Koopa Troopa residents to start kicking other shells when they become Beach Koopas. If a Yoshi holds a Blue Shell in its mouth, it will grow wings and become able to fly.
Blue Shells can be worn by humans such as Mario, which as shown in New Super Mario Bros. turns him into Shell Mario.
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.