Mighty Guard is probably one of the best blue magic for late game. It casts Protect, Shell, and Float on all allies and costs 72 MP.
It's from an ocean creature called Stingray, which is a rare encounter near the Phoenix Tower.
Blue Magic (28/30): Mighty Guard
From Stingray (rare encounter)
This monster spawns in the ocean around the coast between Phoenix Tower and Carwen. It requires Control to cast it on your allies with Learn ability or Blue Mage.
Note that it is immune to Earth and absorbs water, so use different elemental magic until you win.
Stingray also rare-drops a unique item, Dragon's Whisker, which is the strongest whip-type weapon in the game.