To start off, changes for tanks in general:
Increase tenacity effectiveness per point by 0.75x.
Give accessories with the Fending attributes a split amount of Strength and Vitality, if you insist on not making VIT the primary damage stat for tanks.
Now, changes for DRK:
Make Dark Arts boost Blood Price for 10% damage reduction, and/or Blood Weapon for 10% extra damage dealt.
OR
Give Dark Arts synergy with Shadow Wall. When Shadow Wall is used under the effects of Dark Arts, the damage resistance is boosted to 35%, the cooldown is lowered by 40s, and each attack you take inflicts a stacking DoT that starts at 30 and increases by 5 with each additional hit you take. Maybe give the DoT a lifesteal or MP syphon effect. Otherwise, damage resistance boost to 45% with the same CD as before to make up for the MP cost of DA.