Blue Shell

Blue Shells are blue Koopa Troopa shells that first appeared in Super Mario World. This was one of the only games they were worn by Koopa Troopas. The Koopa Troopas wearing Blue Shells were smarter than Koopa Troopas wearing Red Shells. They were also a lot more intelligent than normal green-shelled Koopa Troopas. They simply turn around when reaching a ledge. They also walked faster, and they were the only ones who could kick their own shells. They are a rare type of shell, even rarer than Red Shells.

Like other Koopa Troopa shells, the Blue Shell will 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. During the events of Super Mario World, Blue Shells give Yoshis wings that allow them to fly until it swallows the shell or spits it back out, in which case it will behave like a green shell does, flying out at high speed and knocking over all enemies in its way.

In Super Mario Strikers, as well as in Mario Strikers Charged, Blue Shells appeared 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 him/herself. 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 but 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. The shell version of Mario or Luigi looks much like a Koopa Troopa with a blue shell. When ducking, shell Mario/Luigi cannot be hurt by almost anything. Also, they can Shell Dash. Blue shelled Koopa Troopas that emit blue shell power-ups when stomped appears in multiplayer. It regrows one periodically. A Blue Shell (taking its appearance from Mario Kart: Double Dash), referred to as Spiny Shell, appeared in Super Smash Bros. Brawl as a sticker. If used in the Subspace Emissary, the sticker would increase the launch power of a specific character by eleven.

In Mario Kart
In the Mario Kart series, they are deadly weapons. They are similar to Red Shells, except these home in on the driver currently in first place, destroying anything in its path in the process. However, if the player in first place used it, he/she would instantly be crushed by its own weapon, which is why it's best to use it when the player is NOT in first place. In Mario Kart games made after Mario Kart Super Circuit, Spiked Shells are replaced with the Flying Spiked Shells, most likely because of their low reliability in Mario Kart 64 and Mario Kart Super Circuit. They could be missed and fired at a wall, but would otherwise never fail in hitting the target. They would sometimes crash into a wall, if fired near a curve and were unable to make certain long jumps. These varieties were spiked and called Spiked Shells. In Mario Hoops 3 on 3, this version of Blue Shell appears in the item list of the game. After home in on a player, it will explode.

The flying versions of these were called Winged Spiny Shell. After firing one, it flies over the course, homing on the current contestant in first place. After the shell reaches its destination, it will hover for a brief moment over its target, then smash into the ground, giving off a massive shock wave causing the target and anyone near him to bounce around out of control. Winged Spiny Shell are much more efficient than the previously replaced Spiked Shells. These shells have appeared in Mario Kart: Double Dash!!, Mario Kart DS and Mario Kart Wii. In Mario & Sonic at the Olympic Games, the Winged Spiny Shell appears as an item in certain dream events.

Trivia

 * The blue shell makes an apperance in Super Mario Galaxy in the Sea Slide Galaxy on the level Underwater Cosmic Mario race. When Cosmic Mario grabs a shell its blue.
 * Only Kooper, Parakarry and Kolorado's father wears a Blue Shell without being an enemy.