Blue Magic is basically when you can copy abilites, used in previous Final Fantasy Games, Now in Final Fantasy X it is used as Kimahri's Overdrive - Ronso Rage. You can learn the abilites using the skill "Lancet", which only with Kimahri you can learn abilites, which is his Overdrive. When Kimahri enocounters Biran and Yenke, you want to make sure you use Lancet.

Ronso Rage - Blue Magic
Name	Learned
Seed Cannon	Ragora, Grat
Stone Breath	Basilisk, Anacodaur, Biran
Self-Destruct	Grenade, Bomb
Fire Breath	Dual Horn, Yenke
Aqua Breath	Chimera, Chimera Brain, Yenke
Bad Breath	Malboro, Great Malboro
Doom	Ghost, Wraith, Biran
Trust Kick	Ghost, Wraith, Biran
White Wind	Dark Flan, Yenke
Mighty Guard	Behemoth King, Biran
Nova	Omega Weapon