Been thinking about some lore Justifications for a duty finder blue mage to appear alongside limited blu for sidecontent.

Option 1: The Black Market Blue

This is a simple one, basically after you've progressed a certain amount with BLU, a man appeares in the Ul'Dah backstreets. This man is in the business of selling pre-filled Blue Mage soul crystals to people wanting power. Essentially this is an echo blessed man who has been tagging along with adventurers to learn what he deems to be powerful Blue Magic, then selling the Job Crystal filled up on the Black Marget, some fun oppertunities for some comedy here with they guy basically admitting to being carried by others to learn spells. Upon doing the quest (or maybe just straight up buying it) you get a second BLU job crystal, however as its already filled with spells, its just like any other soul crystals you have for other jobs, at which point you equip it and learn spells as you level like any other job.

Option 2: The Whalaqee

Similar to option 1, but instead of a pre-filled crystal by some black market man, at some point in the BLU quests, a Whalaqee elder comes along to see how Martyn is getting along. Upon his visit he bestows upon you a Whalaqee Soul crystal, the original BLU instead of the copies of the ability that Martyn made. From there it would function like option 1, where you can have spells tied to levels ect, but with this option Martyn could combine your Limited BLU crystal with your Whalaqee one, allowing some minor custimisation to the aesthetics of your skills