So I've been using Dark Knight as a main since I could get the job unlocked in Stormblood. I've really enjoyed using it, but have felt like it needs a bit more love especially with all the Endwalker changes that have been made to its fellow tank jobs. I'll list my thoughts below.
--Dark mind is a magic mitigation, not an overall mitigation. so its a guessing game on what it's useful for mitigating. I'd love to see it be made into a vulnerability down mitigation of 15%.

--All other tanks have a 10 second invulnerability now, such as Warrior or Gunbreaker, so Living Dead in comparison isn't as desirable. Holmgang is a fast mitigation, and has just as much time to use it. Living Dead wasn't given a heal potency buff or blood sucker type ability. Which makes it almost required to have a White mage to use Benediction, or a lot of coordination beforehand.

-All tanks have self-sustain. All of Paladin's, if not most of Paladin's oath gauge abilities provide a regeneration. Gunbreakers now has 2 aurora, and their own form of Excogitation. And Warrior now has Bloodwhetting. Dark Knight is lacking its own form of self-sustain.

-Dark Knight has its Blackest Night as it's only form of self-sustain along with Abyssal Drain. Unfortunately Abyssal Drain is mainly useful for large dungeon pulls, but now shares a timer with Carve and Spit. Which makes it not viable for end-game content. Abyssal Drain does give a set amount of mp like Carve and Spit now after the update, but Carve and Spit doesn't give hp like Abyssal Drain.

-Yes, Dark Knight has Blackest Night. It's seen as the "Best Mitigation in the game". But now after the update, every tank has something that's easily comparable as I mentioned above with the mitigations given to its fellow tanks. Which takes its one unique skill and makes it not as special to the job anymore. Yes it has a shorter cooldown, but you mainly use it for tank busters in later content, or if you memorize the fight and know where it can be viably used.

--Blood weapon is still on a short timer, it could've been given the same treatment Delirium got. If in the case they lowered the amount of stacks you get from 5 to 3, they could boost the mp it gives back to make up for it so there wouldn't be mp loss from a given stack change.

I believe that Dark Knight needs some love because it won't be seen as viable for endgame content as much as the other tanks. It may even be put into a spot where people won't want you to be in a party for endgame content just because of how it may struggle in comparison to the other tank jobs.

Xenosys in his video for the Gunbreaker update said "People seem to forget that you can love something and criticize it, and you criticize it because you want it to be the best and want them to it improve as much as they can." And this is very much how I feel about Dark Knight.