You don't really start to play black mage till 50, really.

Dungeons is pretty much about fire till low mp, transpose till max mp, transpose again etc.
At level 40, it gets a -bit- more interesting as you get your third astral fire/umbral ice stack and at 42 you get firestarter.

Personally, black mage is interesting how to min-max your movement by timing your firestarter/thundercloud procs well and by using aetherial manipulation, especially in movement heavy fights. But this may not be the case for everyone.