Dark Knight could easily use darkness to tank. The old THM from 1.0 actually had a lot of defensive Dark-based spells, which could see a comeback with Dark Knight. For example:
*Dread Spikes - steals HP from enemies that hit you
*Drain - steals HP from enemies
*Sanguine Rite - Restores MP in proportion to damage taken.
*Absorb ACC/ATK/DEF/EVA - Transfers a portion of the target's accuracy/attack/defense/evasion to you.
*Shadowsear - Deals damage and increases your magic potency in proportion to the different in HP between you and the target.
*Punishing Barbs - Channel your pain to an enemy, redirecting a portion of damage dealt to you back to its source.
*Gravity - Reduces the movement speed of enemies within area of effect.
*Sacrifice - Heals a portion of HP and additional HP over time to party members with area of effect.
*Damnation - Haunt an enemy with visions of the hells, dealing damage. Grants the Caster MP in Proportion of to the damage dealt.
*Blood Rite - Spill your own blood in a sacrificial rite, enhancing magic potency.
*Silhouette - Launch a shadowy attack, dealing damage to the target decreasing enmity. Damage type varies with equipped arm. (would obviously need to be reversed to increase enmity).
*Soul Ward - Allows target to instantaneously revive upon being KO'd.
*Shadowfall - Launch an icy attack, dealing damage and temporarily incapacitating the target.
*Emulate - Imitate the very essence of another, assuming the target's elemental resistances.
*Contagion - Share your anguish, transferring all of your enfeebling effects to the target.
*Mass Fear - Causes enemies within area of effect to flee in terror.
*Stygian Spikes - Deals damage over time and reduces the defense of enemies within area of effect.
Someone made a video showcasing 1.0 classes and their abilities, so you can see them being used here:
http://youtu.be/h1m0B_OTjsk?t=11m28s
Admittedly, the old THM also felt very "Red Mage" as it held a lot of utility spells, but I think Dark Knight could definitely take a cue from it and become a very interesting tank.