To learn the spell missile we fight enkidu, who obviously is not natively using missile, he must be casting it as blue magic.

But we can still learn it from him. So are we learning second hand blue magic? Could we learn blue magic from each other? It is interesting to note that we are not allowed to pvp or even duel with blu, is it because we could learn spells like that?

It’s not unheard of, in ff tactics advance if an enemy blue mage used a spell on your blue mage, they could actually learn the spell.