The Blue Shell is a Mario-exclusive power-up that turns Mario into Blue Shell Mario. This grants Mario a Shell Dash, allowing him to duck into the shell while running and slide along the ground like a kicked Koopa shell to take out enemies. He can still jump while sliding, but he has no control over its speed and direction unless he exits it by releasing the run button. It also gives him a Hammer Bros. Suit style duck to repel fireballs. While Shell Dashing, Mario can't create a point combo and will deflect off of most bosses.

Powerups in Mushroom Kingdom Fusion
Mario Universe Super Mushroom | Fire Flower | Raccoon Leaf | Hammer Bros. Suit | Tanooki Suit | Blue Shell | Cape Feather | P-Wing
Sonic Universe 10 Rings | Fire Shield | Kevlar Shield
Arthur Universe Armor | Halo Armor | Sun Shield | Apollo's Shoes | Hermes' Shoes |
Exclusive Sci-Fi Suit | Mega Man Suit | Soldier Suit |
Other 1-Up Mushroom | 3-Up Moon | Poison Mushroom | Starman | Lifejacket | Reaper | Stomp Boots