The third offensive spell-caster job you can acquire; previously the first. It has high MP and Intelligence, necessary for its purpose, while everything else ranges from average to downright pitiful. Like White Mage, it's largely obsolete by the time you get it.
Black Magic is stronger than Spirit Magic and less MP intensive compared to Summon Magic, so you can at least assign Black Mage as a secondary job. However, as a primary job, Wizard covers everything and more. Black Mage has some cool late-game abilities though.
Specialty: Black Resonance
The damage of Black Magic rises with each ally other than the user who has equipped Black Resonance (as a Specialty or support ability). Damage increases up to 1.2 times with a full party of Black Mages.
20% extra damage is fine, but not worth changing your whole party into Black Mages. During late-game, it's worth considering if you're fielding an army of ghost mages.
Level 1: Black Magic Level 1
Fire and Blizzard deal weak fire and water damage, respectively. These spells can be cast on a group, but their power will be reduced.
If you have the Red Mage asterisk, you should know all about these spells. In general, Black Magic is stronger than equivalent Spirit Magic, but the larger the group, the weaker the spell becomes, unlike Spirit Magic.
Level 2: Rod Lore
Unlike Bravely Default, weapon proficiency boosts more than Physical Attack for weapons--and now boosts Magic Attack too, so this isn't an entirely dud ability. Still, the best spell-casting jobs have a rod rating of A or S, so Magic Attack 20% Up is better for upping your damage.
Level 3: Black Magic Level 2
|Command||Thunder: 10 MP Aspir: 1 MP|
Thunder completes the trinity of elemental spells, dealing weak lightning damage. Like Fire and Blizzard, it can be cast on a group, but it's power drops accordingly. Meanwhile, Aspir drains MP from an enemy and cannot be reflected.
Aspir is great to have because it makes your Black Mage somewhat self-sufficient. Don't forget the higher then Black Mage's Magic Attack, the more MP recovered.
Level 4: Black Magic Level 3
These spells are stronger versions of Fire and Blizzard. Except when facing very large groups (more than 3), their damage is higher or roughly equal to Spirit Magic. Against one foe, they're twice as powerful and roughly on par with Summon Magic.
Level 5: Black Resonance
There's not much to say that hasn't been said when this was a humble Specialty. 20% additional damage is OK, but there are far better support abilities like Full Charge, which doubles damage for all spells, not just Black Magic.
Level 6: Black Magic Level 4
|Command||Thundara: 25 MP Drain: 8 MP|
Thundara is the lightning-based variant of Fira and Blizzara. Many foes--in particular weapon wielders, robots and sea creatures--are weak to lightning, making it a choice spell.
Drain damages one enemy and restores the user's HP by the same amount; like Aspir it cannot be reflected. Now your Black Mage can be even more self-sufficient!
Level 7: Black Magic Level 5
To the surprise of nobody, these spells are even stronger versions of Fire and Blizzard. When facing 2 or fewer foes, they even beat out Summon Magic. Trouble is, you can only acquire the scrolls for these spells towards the end of the game.
Level 8: Pierce Magic Defence
The cost for this ability is steep, but it's essential for brute-forcing your way through enemies with high or even maximum Magical Defence. The best late-game party revolves around ghost mages, so you should learn this for when you go up against the aforementioned foes.
Level 9: Black Magic Level 6
|Command||Thundaga: 45 MP Dark: 80 MP|
Thundaga is an even stronger version of Thunder, while Dark deals massive dark magic damage to a single foe. Both are great to have, although it's a shame Dark only damages one enemy. Also, like the previous spells, the scrolls for these are obtained very late.
Level 10: Group-Cast All
This is the second reason to level up a Black Mage. Now you can cast spells like Dark on multiple enemies, although its power will be reduced. Can be combined with the White Mage's Group-Cast Master, but doing so is generally too costly.
Rather, its best applications are for supportive based spells, like Resurrect, Re-raise or any of the Astrologian's buffs. You can use Mist or Blast Spellcraft for similar effects, but that requires expending an additional BP--and BP can become very valuable during late-game.
Level 11: Black Magic Level 7
Flare is essentially a stronger version of the Summoner's Promethean Fire. It deals major fire damage to all foes and cannot be reflected. Sadly, it's also a weaker version of Firaja, making it redundant in the grand scheme of things.