I say they allow blue mage to be strong and have access to all the content a normal class has access to, however give the BLU a crutch that will either for the player to be EXTREMELY skilled in order to be good, or give BLU a DEBUFF or GIMICK that will prevent it from being the end-all-be-all so much to where youll see nothing but blues queing up for dngs.
for example, allow blue to do whatever, but you can just
-give some or all of its skills long cast times, but good mana costs so the playstylee will revolve around timing and positioning your spells.
-or give BLU huge mana drains but a good way to sustain the mana, so they can still have all their skills, but now they have to manage them with the mini game of (Spell cast > spell cast > mana recover > spell cast > spell cast > mana sustain)
just over all something to make the class fun and viable AND NOT RESTRICTED INTO USELESSNESS!!!