Dark Knight seems more of a Damage orianted Tank compared to PLD and WAR.

Thier mechanic heavily depends on Darkside which reduce their damage loss to 5% which allows them to perform more damage than PLD and WAR when it comes to tanking.

Not to mention their Main Tank buff is Dark Dance which is on a 60 second cooldown and provides 30% increase in parry with the addition of 20% increase chance of dodge with Dark Arts.

DRK appears more designed to be a Off-tank than Warrior was but darn can it keep aggro a bit better due to having higher Damage rate than PLD and WAR.