After playing this game since RoZ, I have seen Black Mage have its great times as the job to bring to events and other jobs get pushed aside because they weren't as useful as others. At 75 cap, Black Mage had a great place in many events and for its own solo fun, but now at 90 I have to say Black Mage has lost everything that it had at 75 compared to other DD jobs.
After playing in abyssea for a long time, I felt Black Mage had its own unique traits such as being able to one shot regular enemies easily to farm up azure faster or simply farm pop items. Also Black Mage has the ability to get yellow !! to help get seals and +2 items faster. The power of atmas and cruor buffs spoiled Black Mage a bit too much with refresh and almost double MP.
That all being said, MP cost of new spells for black mage became extremely high and it turned out to be okay in abyssea, but what is bad is outside abyssea, black mages don't provide much damage at all anymore when given a drawn out fight.
I have recently been playing outside abyssea and I have Paladin with Almace/Ochain and let me say, it even out DD's Black Mage if you press a fight where black mages have to rest and get MP after nuking 4 times. Even with 10 MP regen, you are looking at every 30 seconds you get 100 mp back and in that time my Paladin has already obtained 100% tp doing another ~1500 chant du cygne (outside abyssea) and this is not including the regular hits he is doing too whereas a BLM blizzard 5 will do ~ 2500 damage at the cost of 282 MP.
That means you will need to wait almost one minute and 30 seconds to get to nuke again without resting, or rest and do nothing to pull that damage back out which might be around 40 seconds to one minute to regain that MP while melee DD can just keep outputting at a much higher rate than you due to such a limited MP pool outside abyssea which makes convert half effective because your MP is halved.
What is depressing is my Paladin was the comparison to a Black Mages damage, and right now people are begging for Paladin to be better...
All that being said, TLDR version, Black Mage needs some sort of love where the cost of spells are concerned. Their MP pools are way too low for the cost of the new nukes, and even old ones can't keep up with the new level 90 items for melee jobs. Hope there is something done about it, because everytime I play Black Mage at an event that doesn't require Sleepga (which is rare now), I feel like I could be doing a lot more for the group.