I don't really see why blue mage couldn't just learn spells from monsters. In XI, you learned weaponskills and abilities and traits automatically while leveling up. You learned spells by buying them from item shops or as quest rewards, each time just using a scroll to learn the spell. They implemented a completely new system for blue mage alone. I think saying they couldn't do that just because it's not how the game currently works isn't a good argument because that was the situation XI was in before it got blue mage implemented.

They could do it if they wanted. Whether they want to or not is another story.