The Leveling Problem explained.

[Arohk]

It's very strange considering how popular the game is.

No , you are correct, the Black Mage job is very unintuitive and badly explained in the game. Also the gameplay style changes completly while leveling.

first you must switch from fire to ice via Transpose, then you get Fire 3 but cant actually use it unless you get a freecast, but the freecast proccs at the very end of a fire cast so you have 0.5 seconds to react and use it, then you get Ice 3 to switch from fire to ice and don't use transpose anymore at all. Also you dont use Fire3 freecasts anymore, you only use it to refrech henochian at the end of the fire rotation.
And very much later you get Umbral Soul which seems useless at first glance but before you get Umbral Soul BLM feels extremly clunky in any fight with downtimes.
The job is fine on high levels but feels totaly trash on low levels.