Archer requirement to become Black Mage?

[Lemon8or]

That's why for every job the player usually has to level both subclasses to 34 in addition to leveling the base class to 50.

Not entirely true. Some classes need to be higher like Pugilist to 42 to unlock Mantra to cross-class. Some classes need lower like Marauder to 26 to unlock Mercy Stroke.