Regardless of their lore and "learning" mechanic, the spells won't deviate much outside the current tool kit of their role. If they're casters, they will fundamentally have spells that are similar to BLM/SMN/RDM, e.g. direct damage, dot, procs, etc. The "learning" aspect will likely occur inside of job quests rather than in the open world (for obvious reasons) and involve "get hit with X attack Y times" or whatever. I doubt BLU will be a tank because the word MAGE is in their name and mages wear casting/healing gear and tanks wear fending gear. It would look weird for a MAGE to be casting spells wearing THIS: .