Dark Knight experienced a lot of changes in 4.0. Many skills were lost to the new role actions, and the entire way the job functions was reworked. The skills of 4.0 lack synergy with the rest of DRKs toolkit, and optimizing the class is currently unrewarding, as the skill floor and skill ceiling feel inches from each other. Dark Knight has many options, but those options all equate to roughly the same result. Making the class feel stale. Personal mitigation also took a large hit, and DRK is noticeably squishier than the other tanks.
Looking at 4.0 skills, Square Enix seems like they have a plan for the way DRK should play, and I think the changes I'm about to suggest fall in line with that vision.
Grit cooldown changed to 10s
Dark Mind:
Reduces damage taken by 10%.
Can only be executed when succumbing to the Darkside.
Duration:15s
Additional effect: 30% chance that upon taking damage the next Dark Arts will have no MP cost. Duration: 10s
Dark Arts Effect: Increases damage reduction to 20%.
Dark Arts fades upon execution.
Blood Price:
Restores partial MP when damage is taken.
Duration:15s
Additonal Effect: Increases Blood Gauge by 1 immediately and another 4 over time while in battle.
Additional Effect: Halves the Blood Gauge cost of Quietus and Bloodspiller.
Duration: 15s
Can only be used while under the effect of Grit.
Bloodspiller:
Delivers an attack with a potency of 400.
Additional effect: Execute next combo without meeting combo prerequisites.
Dark Arts*Potency:540
Grit*Potency:475
Dark Arts+Grit*Potency:650
Blood Gauge Cost: 50
Dark Arts fades upon execution.
Dark Passenger:
Deals unaspected damage with a potency of 100 to all enemies in a straight line before you.
Can only be executed when succumbing to the Darkside.
Additional Effect: Stun 5s
Dark Arts*potency: 240
Dark Arts Effect: Damage over time. Potency is reduced by 5% for the second enemy, 10% for the third, 15% for the fourth, and 25% for all remaining enemies.
Potency: 40
Duration: 15s
Dark Arts fades upon execution.
The Blackest Night:
Creates a barrier around self that absorbs damage totaling 20% of your maximum HP, or around a party member that absorbs damage totaling 10% of your maximum HP.
Duration:*15s
Dark Arts Effect:
Extends effect to nearby targets.
Increases Blood Gauge by 50 when full 20% (10%) is absorbed.