I was wondering if a straight up Blue mage class would work or if it can't be implemented as for the same reasons as Red Mage.

Currently the Blue Mage's shtick is Blue Magic, something that allows then to use enemy skills, however because of this they would propbably exceed other class in the skills they can learn, use and equip. Also because the skills range from healing (White wind) to DPS (1000 needles) it would break the trinity a little (Of course if no tanking blue magic is introduced it could work as a medic counterpart to Warrior, which was probably meant as a DPS-Tank hybrid). Though the skills would probably be in the crossclass system

On the other hand if it winds up being a Job to say, Gunner or musketteer, Blue Magic would likely be out of the question but the concept would still persist by having the character kill monsters with a certain ability (Like a quest with 1000 needles as the reward requiring you to kill cacutars.)