The only way I see Blue mage to be implemented into a system like we currently have is have a set amount of abilities like any other job class currently. These are let's call it a Rage skill in a natural state when you first unlock the job were if you are hit/seen a spell enough times or whatever is deemed learnable by SE you earn that spell and is place inside Rage skill book. Once learned you unlock that natural state ability to be able to attach spells/abilities from the Rage skill book, if you want to learn any new spells though you will have to put the spell/ability back into its Rage/Neutral state by unattaching the spell/ability selected and repeat the process since well it is blue mage after all.

Certain abilities obviously will be locked to increase blue mage spells abilities etc., but the others would use the Rage system to attach spells learned from your foes or what deemed learnable. What the actual blue job entails is up for debate for me personally though would like hybrid dps/support role not so much in healing role, but maybe help increase the output of healer % of healing spells for example. Well this my personal opinion on how maybe the job could be introduced into the game and I understand this is a simplified version and more depth would be involved into a system like this thought it would be interesting at least in concept lol.