Blue Shell

Blue Shells are blue colored shells that Koopas wear.

In the Mario Kart series Blue Shells with spikes on them and wings appear, however named Spiny Shell.

Super Mario Strikers had Blue Shells as an item. If the Blue Shell hit anynoe, the character who got hit, would freeze from some time.

In New Super Mario Bros., Mario or Luigi can become Shell Mario or Shell Luigi by getting a Blue Shell.