If i had any complains about Dark Knight, it would be the following Abilities even being a thing (warning: unpopular opinions incoming): The Blackest Night and Delirium.
I dont think i need to explain any reason why anyone sane would not dislike Delirium, as mentioned already, its just Inner Release in disguise and that thing belongs to either WAR or into the trash. Personally, i'd rather wish that Dark Knight had something more unique instead of this. If i had to make a suggestion, then i would say to replace it with something similar to Last Resort from FFXI, where the DRK gets +15% Damage up and -15% defense (can only be used below 50% HP and with a somewhat long cooldown, to balance it out a little) which lasts until either the DRK dies or goes back to over 50% HP.
as for Blackest Night, i think it isnt a Ability a Dark Knight should have. It's not that is by any means a bad Ability, but it is something that conflicts with the Ideology of the Job. Instead of it being a Shield, make it a Buff that can be used on a allied target for which the Dark Knight is able to take the full damage of the next hit once during its duration.
"But Anhra, a tank is suppoest to stay alive and not to get himself killed with his own Abilities!". I know, but hear me out on this. My point with those kind of ideas is, that Dark Knight's gimmick should be something like "gets stronger the closer to death it is and that Power comes at a prize". This could be backed up further by a trait called "Desperate Blows" for which the DRK gets increased dmg when low on HP at the cost of having a longer GCD on the basic Rotation (but also stronger MP/HP regen effects from Syphon Strike and Soul Eater).
