I feel like it's kit is sort of incomplete, and it's abilities are underwhelming in a few ways... Possible changes that I think would help the job a bit:
Cross classes: I feel like Lancer sub would be perfect on Dark knight... Keen flurry, and Invigorate would help Dark knight immensely. I would say lose Gladiator, but we need provoke(Convalescence is nice too ). Marauder also has bloodbath, foresight, and Mercy stroke(very nice when combined with sole survivor). A way to make this all right would be to add the provoke effect to something like plunge(with Dark Arts maybe?). Or just lose the MRD sub.
Blood Weapon: Gives you 10% speed bonus, which in turn causes TP to run dry. It's a big problem as an OT. This could be fixed by changing it so that TP costs are reduced while it's in effect. Either that or change the speed bonus to a damage bonus, and slightly increase the MP return per hit.
Living Dead: A bad skill, I almost forget it exists sometimes. It stresses the healers far more than Hallowed ground or Holmgang, and is almost never a good idea to formulate a strategy using it. As of now, if you're in a PUG, it is pretty much a death sentence once walking dead is activated. It can work in very organized groups, but it draws a lot of resources, and with what other tanks bring to the table, why bother? There are a lot of ways to fix this skill.
Soul Eater: Good skill overall, but I feel like the HP return should be slightly increased, seeing as it's one of our main mitigation tools.
Dark Dance: This skill is insanely good for large trash pulls when used with Dark Arts, but is a bit unimpressive when used against boss mobs. I'm honestly not sure what to fix on this(or if it needs fixed at all), but a lot of people say they would like it to increase Parry more with Dark Arts, instead of adding evade chance.
Shadow Wall: Feels a bit underwhelming compared to the same tier of skill the other tanks have... I feel like it should have a Dark Arts variation. Or just increase the duration/decrease the cooldown.
Dark Passenger: Amazing skill for large trash pulls. Needs to lose the MP cost though. DRK is already very MP hungry, and this skill doesn't help. Or keep the MP cost and add an enmity boost to it.
Carve and Spit: I don't know. It feels pretty weak when used without Dark arts for the MP restore. Pretty good when used with Dark Arts though. Maybe add a minor TP restore on the non-Dark arts version.
Blood Price: Pretty meh when used against a single target. I feel like it needs a way to be more useful while tanking a single enemy(Maybe w/ Dark Arts).
Delirium: It's okay... But I feel like it's a little underwhelming for a combo finisher. Maybe add a damage buff of 10% or so similar to maim? Or have it restore the same amount MP as Siphon strike so it could be an MP return combo(and not be useless when a monk is around).
Dark Arts: MP cost is steep, but that's okay, it makes you be more aware when playing Dark knight. As you can see above, I think it should be more active in Dark knight's kit. It's the main gimmick of the job, and I feel like it isn't used enough.