https://www.polygon.com/2019/1/15/18183822/final-fantasy-14-blue-mage-how-to-unlock

Final Fantasy 14’s newest job, Blue Mage, dropped today as part of patch 4.5. Unlike the other jobs in the game, Blue Mage learns spells from enemies, requiring players go out on hunts to collect them.

The Blue Mage is considered a “limited” class, which means it can’t be taken into dungeons, trials, or raids through normal party-finding queues. You can take Blue Mage into these types of content in pre-made parties, however.