A Blue Shell is an item featured in multiple Mario games that has varying abilities depending on the video game. In the original title (Super Mario World for the SNES), its only distinct purpose was giving any Yoshi the ability to fly for a short time when eaten.
Mario Kart (series)
In the Mario Kart series, Blue Shells are arguably one of the more preferred items and for the victim one of the most annoying. Rather than being just regular Koopa Shells colored blue, they feature spikes and, more recently, have wings as well. When thrown, it'll head towards the racer in first place and attack them. In the earlier titles, the Blue Shell would not only attack the person in first place, but would also plow through any enemies in front of the racer who threw it. Recent titles have implemented ways in which you can dodge the Blue Shell.
To avoid the Blue Shell in Mario Kart games, turn invisible using Boo. This makes it more difficult for the Blue Shell to see you. Or turn invincible using Star or get powerful using Bullet Bill. If you don't get it, come 2nd and fool your rivals coming 1st. This will protect you from this annoying thing. In DS version, karts with high drifting can avoid it. Because after drifting with red/blue sparks, release it. When you made mini-turbo, hop at the right timing to avoid blue explosion. This is really difficult because blue shell will immediately aim at you.
Super Mario Strikers (1+2)
New Super Mario Bros.
Blue Shells also appeared in New Super Mario Bros. where they could actually be worn by Mario or Luigi. They would also allow the user to travel faster than before and ram into enemies just as a kicked shell would do. The user would be able to permanently kill several enemies that wouldn't be able to be permanently defeated before without the help of a star, like Dry Bones , Giant Dry Bones, Thwomps, and Wigglers, to name a few. The blue shell also gave the user better underwater agility and immunity to enemies and spikes as long as the user was tucked in the shell.
- Shells - Other types of shells, most of which are from the Mario game series.