I have my Black Mage around level 60 I believe.

What I can tell is in order to play Black Mage you have to learn a quite in depth and complicated skill rotation. (At least for me it is.)
Black Mage is considered the turret mage so it requires you to stand on one spot as much as possible.

Also Black Mage lacks mobility by a lot and is extremely squishy.
So a boss AoE attack will hurt extremely much if not avoided.