Blue Shells are blue Koopa Troopa shells that first appear in Super Mario World. This is one of the only games where they are actually worn by Koopa Troopas. The Koopa Troopas turn around when reaching a ledge. They also walk faster. They are a rare type of shell, rarer than Red Shells. It can make any Yoshi fly instantly upon putting one in its mouth.
Like other Koopa Troopa shells, the Blue Shell can be launched when the Koopa Troopa wearing it is jumped on. It can also be held and thrown. There are also flying Red Shells which appear in Super Mario RPG: Legend of the Seven Stars. They are not to be confused with Parabuzzy or Buzzy shells, which are darker and more spherical.
In Super Mario Strikers, as well as in Mario Strikers Charged, Blue Shells appear as items. They are usually collected in a pack of three. When used, they will be fired at the opposing players, however, they do not home in, like the Red Shell does. If the Blue Shell happens to hit a player, they will instantly freeze in one spot. This will render them unable to move, and, thus, unable to get the ball. The effects wear off after a few seconds. Unfortunately, these shells can freeze both the users teammates, or the user. If the shells fail to hit anyone, they will continue bouncing around the stage until they either hit a player, or just wear out on their own. A more effective version of this item is the Giant Blue Shell.
In New Super Mario Bros., there is a Blue Shell item. It is not used as a weapon, but as a power-up. The shell turns Mario or Luigi into Shell Mario or Shell Luigi respectively. When ducking, Shell Mario/Luigi is invulnerable to almost anything. Also, they can Shell Dash. Blue-shelled Koopa Troopas that leave Blue Shell power-ups when stomped on appear in multiplayer. They respawn periodically. Some obstacles can also only be passed by using the unique Shell Dash the power up offers. If a single Brick Block is protected on the top, bottom, but not the sides, the Shell can smash it allowing access to otherwise inaccessible areas.
The Blue Shell appear in Mario & Luigi: Bowser's Inside Story, as an item found in the overworld and once in the Energy Hold in Bowser's Body. Mario can go in Blue Shells (and turn into Shell Mario from New Super Mario Bros.) by striking Blue Shell blocks. Luigi can then hit Mario using his hammer, and this will make Mario go into the shell, spinning straight forward. This can be used to hit blocks in areas which Mario and Luigi can't directly go into. It behaves similarly to Kooper and Koops's abilities in the Paper Mario series.
 In Spin-offs
In spin-off games, such as Mario Kart, there are blue colored shells with spikes, which are dangerous power-ups. They were called Spiny Shells in Mario Kart 64 and Mario Kart Super Circuit. Spiny Shells act similarly to Red Shells, as they knock over the target in front of the player who used it. However, it targets the one in first place, where the shell bowls over the kart and gets destroyed. Although a reliable item, the shell stops working if it hits a wall. In Mario Hoops 3-on-3, this version of the Spiny Shell appears in the item list of the game. After homing in on a player, it will explode, affecting anyone within the blast radius.
In Mario Kart: Double Dash!!, the Spiny Shell had gained wings, allowing the item to fly over the course while avoiding all the obstacles around, as opposed to its former appearance, when it is destroyed when it hits an obstacle. After firing one, it flies over the track, homing in on the current contestant in first place. Once it reaches its destination, the shell hovers briefly over its target and then lands on the kart, blowing up everything in an area around the kart in an explosion. This winged version of the shell has appeared in Mario Kart DS and Mario Kart Wii. In Mario Kart 7, however, the Spiny Shell is reverted to its wingless form, though now it will bowl through anything in the course to reach the first place person.