Dark Knight has always been a job that played around their HP in some way. Either by sacrificing HP to enhance their attacks, or draining it from enemies. With the exception of Abyssal Drain... that entire identity is missing from the current DRK. I'm sure that concept is hard to work into a tank but here are some ideas I had on how to fix that to not only make DRK more enjoyable and engaging to play, but also fix the squishy issue in dungeons.

New toggle Ability:
Drain/Aspir: A new action that augments certain weaponskills. In Aspir mode, weaponskills and effects that currently restore MP continue to do so. In Drain mode, however, those effects shift to a HP recovery effect instead. This plays into Dark Knights identity without really changing anything drastically. It also gives DRK sustain when it needs it in Drain mode, or more DPS in Aspir mode to regain mp for more weaving. This would effect Delirium, Bloodweapon, our AoE combo, and Abyssal Drain + Carve and Split. Capped on HP? You can use Abyssal Drain AND Carve and Split as MP recovery tools. Need HP? You can now use both as HP recovery options!

Keep Abyssal Drain and Carve and Split as a shared timer, but give them 2 charges. Abyssal Drain could be a massive MP recovery tool in Aspir mode to burn things down faster with extra Flood of Shadows.

The Blackest Knight: No MP cost or at least reduced MP cost to get a net profit on a fully broken shield.
Trait upgrade at lv.82 with additional Effect: Dread Spikes
Dread Spikes: Restores HP with a potency of 600 once the shield expires or breaks.

Dark Mind: Changed to -10% damage for party member. Upgraded to Dark Missionary at 76 -15% damage on target party member. Remove Oblation as it's just button bloat.

Blood weapon: 5 stacks on use, easy enough.
Enhanced Blood Weapon at lv.82: Restores both HP and MP while the effect is active. Increases damage of weaponskills while under the effect of Blood Weapon by 10%

Living Dead: Effect changed completely. Summons your simulacrum to cloak you, similar to Reaper's Enshroud, giving you a massive shield. Double down on the shield aspect that Dark Knight has currently with TBN to make it feel different enough as an invuln, while still filling what the ability needs to do. Paladin you are immune to pretty much everything, warrior you holmgang and heal yourself back up with raw intuition/bloodwhetting, gunbreaker you drop to 1 hp and you're good for 10 seconds, Dark Knight you put on a massive shield for yourself to survive on.