Update: Square Enix offered up more details about the Blue Mage and the unlock requirements. To unlock the job, you have to have a Disciple of War or Magic up to Level 50 and have completed the all of the main story quests of A Realm Reborn. No expansions are required to access the Blue Mage. There is a caveat though: Blue Mages can't queue for Duties requiring matchmaking. That means no Duty Roulette, no PVP, and no Deep Dungeon. Instead, they can only access instanced content with a preformed party. To make up for being mostly stuck with solo content, the job gains more experience than other jobs in open-world content.
Today during the Final Fantasy XIV Fan Fest 2018 keynote, Square Enix president Yosuke Matsuda joined the stage to announce a new job coming to Final Fantasy XIV: the Blue Mage. A short video showed the classic Final Fantasy learning several actions and abilities from monsters in the game. Unlike most additional jobs in Final Fantasy XIV, Blue Mage isn't coming with an expansion. Instead, it'll be launching in the upcoming Patch 4.5.
There are several caveats. The first is Blue Mage is the first "Limited Job" in Final Fantasy XIV, not attached to any existing class. The second is that the level cap for Blue Mage is set at level 50 and will be raising in future patches. All Blue Mage action are learned from monsters, and the team at Square Enix will be diving into the details of that in tomorrow's Letter From the Producer Live.
Patch 4.5 comes ahead of the planned release of Final Fantasy XIV's next expansion, Shadowbringers. Shadowbringers is coming some time in Summer 2019.