This is more for fun than any serious implementation into the game.
While it's true that Dark Knights in other Final Fantasy titles would sacrifice their health for power, I've noticed that many of the abilities would also involve hitting all enemies. And with this tier having so many instances of needing to disengage the boss, it gave me an idea.
My goal for this was is increase the interaction between the Dark Knight offensive kit as well as break up of the monotony of the 1 minute burst phase.
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Dark Arts: A channeled ability can turned on/off. While Dark Arts is active, your Weapons skills (Hard Slash, Syphon Strike, Souleater, Bloodspiller, Delirium combo) emit a wave that extends the reach of the attack to 10 yalms in a line aoe which deals full damage to the first enemy hit and 50% less to all remaining enemies. Each weapon skill used under Dark Arts will consume 1000 MP.
Salted Earth: No Longer give Salt and Darkness. Cooldown reduced to 40 seconds. While Salted Earth is active, all enemies standing in it will receive damage with a potency of 100 upon execution of weaponskills activated under Dark Arts.
Delirium Combo: now 40 second cooldown to help with MP economy and give more to do outside of 2 minute burst window
Flood of Shadow and Edge of Shadow will be merged so that the first enemy hit will take damage equal to a potency of 460 and the remaining enemies will be hit for 35% percent (similar to Shoha and Shoha 2)
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The idea came from playing FF16. If you used Lightning Rod, each instance of damage at the lightning rod would emit an aoe to nearby enemies. When combined with a move like Gigaflare, which deals continuous damage in a beam, this massively increased the damage output. The closest thing that FF14 has to that would be the White Mage's Lillybell, but I'd imagine this would be super complicated to implement so I used Ninja's
Doton/Hollow Nozuchi as a way it could be done.
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Regarding The Blackest Night, one of the perceived strength of TBN and Dark Knight as a whole is its flexibility. I believe these changes to Dark Knight's defensive kit would reinforce that.
The Blackest Night: No longer cost MP. When TBN is completely absorbed, Cooldowns for Dark Mind, Oblation, and Shadowed Vigil/Shadow Wall are reduced by 15 Seconds.
Dark Mind: Now reduces damage by 10% and reduced Magic damage by 10%. Also grants a regen for 15 seconds
Oblation: Reduces Damage by 10% as well as granting Shadowskin, which is a barrier equal to 10% of the target's maximum HP. Still two charges.
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Regarding Dark Knights in Dungeons, it's fine. I've never had problems with survivability. If sustainability is an issue in dungeons, the best I could suggest is making Abyssal Drain a spell that cost 3000 MP. But Dark Knights have plenty of options to survive.