Blue Shell

Blue Shells, also known as Blue Koopa Shells, are blue Koopa Troopa shells that first appear in Super Mario World. While occasionally worn by Koopa Troopas, they are more prevalent as items that Mario or Luigi can use.

Super Mario World
Blue shells first appear in Super Mario World, either alone or on the back of a Koopa Troopa. As with other colors of shells, jumping on a Koopa Troopa will cause the shell to be removed, where it can then be held or thrown. When Yoshi eats one, he will grow wings and gain the ability to fly for a limited time, at the end of which he will swallow the shell. Blue Yoshi can use this ability by eating any color of shell.

New Super Mario Bros.
Blue shells appear again in New Super Mario Bros., this time as a power-up. It turns Mario or Luigi into Shell Mario or Shell Luigi respectively upon contacting it. By using the Blue Shell, the player gains the ability to shell dash. While ducking or shell dashing, Shell Mario or Shell Luigi are invulnerable to almost anything, excepting boss attacks and falling into a pit, poison, or lava. They can also Ground Pound on Spinies to send them spinning in their shells, similarly to a Koopa Troopa when kicked. Additionally, the power-up allows them to swim faster and with more precision, similar to the Frog Suit from Super Mario Bros. 3.

The item can be collected by defeating a wandering Hammer Bro, stomping the Blue Koopa Troopa found in the Ice Stage of the Mario vs. Luigi mode, hitting a Flying ? Block, visiting a Red Toad House, or in a Roulette Block. However, it cannot be found in any of the normal ? Blocks in the game.

The New Super Mario Bros. Player's Guide has an icon for Blue Shells in the legend for level maps, but it goes unused, as Blue Shells are not found in normal ? Blocks.

Super Mario Galaxy
In Super Mario Galaxy, if Cosmic Mario picks up a Koopa Shell during his race in the Sea Slide Galaxy, it will turn into a blue-colored shell.

Super Mario 3D World
Though the Blue Shell itself does not appear in Super Mario 3D World, the player can jump on a Koopa Troopa and steal its shell. A similar form of shell dashing exists in this game, but only with this held item and not as a power-up form. When shell dashing as Toad or Rosalina, the shell's color turns dark or light blue respectively, similar to the Blue Shell in New Super Mario Bros.

Mario Strikers series
In Super Mario Strikers and Mario Strikers Charged, Blue Shells appear as items; in the latter game they are cyan and frosted with ice instead. They can be collected in a pack of three. When used, all three are fired at the opposing players, but do not home in like the Red Shell does. If a Blue Shell hits a player, they will instantly freeze in that spot, rendering them unable to move. The effects wear off after a few seconds. The shells are able to hit and freeze both the player's and opponent's team members. They will continue bouncing around the stage until they either hit a player or disappear after a set amount of time.

A more effective version of this item is the Giant Blue Shell, which appears individually, and is able to run over players while continuing to move.

Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
The Blue Shell appears in Mario & Luigi: Bowser's Inside Story and its 3DS remake, as an item found in the overworld and once in the Energy Hold in Bowser's Body. By striking Shell Blocks, Mario can go in Blue Shells and turn into Shell Mario. Luigi can then hit Mario using his hammer to make Mario retract into the shell, spinning straight forward. This can be used to hit blocks in areas that Mario and Luigi cannot directly enter. Because the shell is so heavy, Mario cannot jump while wearing it. If he attempts to jump, he tosses the shell off of his body and it disappears. This serves a similar purpose to Kooper and Koops' field abilities in the Paper Mario series.