Blue Knight's element is Ice, which immobilizes an opponent and leaves them in a frozen state until they break out of the ice or get hit by anything. His Splash Attack and Air Projectile, "Ice Shards" are useful to immobilize groups of enemies immediately, dealing a good amount of damage; His Magic Projectile, "Ice Bolt" is good for individually freezing an enemy, with it also dealing a good amount of damage; His Elemental Infusion, "Ice Encasement" is great for infinitely combo locking an enemy until the enemy gets beheaded if done correctly; His Magic Jump, "Ice Fist" is good for immobilizing an individual enemy, like "Ice Bolt", except the Blue Knight has to get up close to do so and it boosts the Blue Knight into the air. The only main downside to Blue Knight is that his Splash Attack and Air Projectile are expensive, only allowing 2 consecutive spells before needing to recharge fully to do again; Pink Knight and Iceskimo share this downside with their Splash Attack. Due to Blue Knight's exclusive abilities and perks, this considerably makes Blue Knight one of the best characters in Castle Crashers. In addition to this, Blue Knight arguably has one of the greatest unlock paths out of any character in the entire game, rivaled only by the other starting knights.
Damage: Base Magic Damage (59 damage; 75 at max potential)
A pointed bullet of ice. The fact that this freezes normal enemies makes it an extremely good projectile attack. Some enemies can resist its damage and freezing effects, though. Also, this spell will always do damage even on frozen enemies, which will unfreeze them.
Damage: Base Magic Damage x 0.5 (30 damage; 38 at max potential)
Blue Knight uses his splash attack for his A+RT+Y spell, but he still gets the kickback boost from it. It retains the cost, but you don't need to have multiple shots to stay in the air for a long time, just use Fly (A+X --> YY; X Y Y (Repeatedly)). This attack is probably the determining factor in Blue Knight being considered to have the best crowd control in the game, because you can use it in the air. It can potentially save your life when using it on the ground might screw you over.
Damage: Base Magic Damage + Base Melee Damage (96 damage, 110 at max potential)
Use / to perform. This is considered the best Elemental Infusion combo in the entire game, and for a good reason. This explanation gets into the technical details to teach you about some of the complex subliminal game mechanics that can potentially improve your overall skill.
When enemies are frozen, their hit counter will be reset. This means that even after unlocking the Level 50 Combo (XXXYY), using it is only a waste of time for Blue Knight and Iceskimo, since they don't need that second Y in the Level 50 Combo to knock the enemies over and reset the hit counter because it's already reset by the Ice. Blue Knight and Iceskimo can therefore repeat this infinitely, and they're the only two characters that can do this. Everyone else needs to use the drill spin to reset the hit counter.
Since the combo can't be interrupted by anything except for straggling enemies and other players; that means that the damage is actually as high as the health of the enemy with the most health that can still be combo locked, which is the BeefyBrute, who has 4,500 HP on Insane Mode, meaning that the max damage is 4,500 when being used as an infinite.
If trolls are used as an example, then the max damage is infinity, as trolls can't be beheaded like every other regular playable enemy, so the combo can continue forever.
One can conclude that the Ice XXXY is either infinitely better than all of the rest combined, or least 20.45x (4,500/220) better than the next best, which is Non-Elemental.
Damage: Base Magic Damage (59 damage, 75 at max potential)
A giant fist made of ice punches upwards, boosting you into the air. Freezes any enemies that you are overlapping when used. Note that it will do damage to frozen enemies and thus unfreeze them. This magic jump is rather awkward to start up a juggle with as every other character can start a juggle immediately.