...wait, I just did a post on Summoner, why does Black Mage need adjustment?
The answer is... it SORT OF doesn't, but... then you start looking at its paradigm, and the way it's built versus Red Mage and its intended melee contemporary, Samurai.
Let's get one thing out of the way. As it stands, Black Mage's damage, while considerable and indeed higher than Red Mage's isn't enough to justify its lack of utility. Sure, I spent a lot of time whinging about Summoner's lack of damage in the other topic, but let's just go over some things.
Black Mage AOE is perfectly fine. That's all I need to say. No need for adjustment, it is far more powerful than its counterparts in Red Mage and Summoner. As much as I'd like SMN to be able to catch up for AoE, Black Mage would be fine at least keeping this. Flare didn't need the dropoff nerf, though. Moving on.
Black Mage lacks power for its lack of mobility. This is technically a trapping of both casters and healers, but with Red Mage having it mitigated with Dualcast, and Summoner having mostly DoTs and Ruin II, Black Mage is in a strange place where its melee counterpart not only performs better than it, but also has less flaws. To improve this, I propose that Black Mage has their single target damage improved. In fact, I would dare say that either Fire IV and Blizzard IV ought to be buffed to a whopping 300 (technically 594 after Astral Fire III and Enochian for Fire IV) potency, Thunder III have a boosted potency, or the addition of Astral Fire IV, for a full 2x damage boost. Astral Fire IV seems like the more balanced adjustment, since it would put Fire IV at a practical 572 instead of 594, but it would still be an enhancement. Umbral Ice IV could simply allow instant cast for Fire III. Also, it wouldn't hurt if Thunder III was treated as an ability when players are affected by Thundercloud, so it wouldn't disrupt our Fire rotation. With that said, Foul could also do with a slight boost, depending on Astral Fire or Umbral Ice, with Astral Fire giving a damage over time effect and Umbral Ice giving it 50 potency per stack, or the other way round.