Blue Kirby is an alternate coloration of Kirby. This alternate coloration is also used for Kirby in Super Smash Bros., Super Smash Bros. Melee, Super Smash Bros. Brawl, Kirby Air Ride, and Kirby 64: The Crystal Shards (as the 3rd player in minigame mode). It is actually unknown if this is an official coloration as these have only been used to tell the first player from the second, but Kirby does turn blue when using the Ice and Freeze abilities.
Blue is also a Spray Paint pallette swap for Kirby in Kirby & the Amazing Mirror and Kirby: Squeak Squad, only it is called Ocean. The dark blue spray paint is called Sapphire. The latter resembles Meta Knight's coloring.
Meta Knight is not confirmed to be a Kirby although he has a similar face to Blue Kirby.
In Kirby Mass Attack, Kirby will turn blue when he is about to die.