I want to revise my list of changes from the OP here, factoring in the fact that SE is very unlikely to actually add or replace whole skills outside of the expansion's initial patch. They've never done this before so I don't expect them to add Scourge back or give us a new defensive CD. So realistically, with that in mind:
Defensive:
Dark Mind:
Reduces magic vulnerability by 15% for 10s.
Dark Arts effect: Reduces all damage taken by 15% for 10s.
-This makes DAing this ability extremely worthwhile. For magic tank busters, you can probably keep it unboosted, but in between tank busters it becomes a really good cooldown for tanking out of stance, mitigating cleaves and AAs, etc. and still comes at a cost of DPS to balance it out.
Shadow Wall:
Reduces damage taken by 30% for 20s.
-As stated in the OP, this makes Shadow Wall unique and gives it its own strength compared to Vengeance and Sentinel.
-Alternatively, if we wanted to make it more like Vengeance (shorter recast, weaker, but with a DPS benefit...)
Shadow Wall:
Reduces damage taken by 30% for 10s. Recast time 150s.
Dark Arts effect: Gives 50 Blood Gauge.
Blood Price:
Restores partial MP when damage is taken for 15s and reduces damage taken by 10% (or increases parry rate by 40%)
Additonal Effect: Increases Blood Gauge by 1 immediately and another 4 over time while in battle.
Can only be used while under the effect of Grit.
-Since Blood Price is now locked behind Grit there is very little reason why it should not have a defensive benefit in the same way that Blood Weapon has an offensive benefit. If people feel this is unbalanced, I would be fine with them having the two skills share a cooldown to add an additional trade-off mechanic to the job, especially when factoring in a Blood Weapon change I've outlined below.
Abyssal Drain:
Deals unaspected damage with a potency of 120 to the target and all enemies near it.
Dark Arts effect: Restores 1000% of the damage dealt as HP, divided by the number of targets hit.
-This makes Abyssal Drain an excellent Clemency equivalent. It cuts into our DPS just like Clemency does (or how Equilibrium does due to being Defiance locked) but provides us with snap-sustain. If this feels too unbalanced, it can keep its original 100% heal on every target while out of Grit, and lock the distributed 1000% effect behind Grit.
If these defensive buffs seem like too much in light of TBN, I would be okay with them increasing TBN's recast to, say 20 or 25s to make it less spammable. I feel like this would be a good QoL thing as well to reduce APM bloat. The power of and over-reliance on TBN right now makes our entire defensive cooldown suite feel incredibly lop-sided. The job doesn't feel good anymore defensively. It completely lacks the ability to smooth out incoming damage consistently and is extremely stop-start-stop-start with its mitigation as opposed to being able to level out the damage evenly and in a way healers can properly account for. TBN's stop-start nature makes our damage-taken unnecessarily spikey. We'll take the full brunt of incoming damage on our bare ass for 10s and then take no damage for 5s, as opposed to our HP just gradually falling slower in a way that healers can predict and deal DPS around.
As far as DPS....
After some thought, I don't think damage over time should be added to Bloodspiller or unboosted SE, because it would make our rotation unnecessarily rigid in terms of when you can use these abilities in terms of not clipping the DoT, so here is what I thought of instead.
Dark Passenger:
Deals unaspected damage with a potency of 150 to all targets in a line before you.
Additional effect: Damage over time with a potency of 70 for 15s.
No Dark Arts effect, MP cost returned to 1200.
-Because of the defensive buffs above and the fact that the blind still somewhat clashes with Blood Price, I decided to remove the Dark Arts effect. In exchange, the ability is now returned to its old MP cost, same recast time, and has the same total potency as Scourge. (150+(70*5)=500)
This would bring DRK's DPS up to an acceptable level to keep it competitive with PLD given its lesser utility.
Alternatively, they could do either of these instead:
Blood Weapon:
Additional effect: Increases Damage dealt by 15%.
-or-
Delirium:
Additional effect: Extends the timer of Blood Weapon by 8s and increases damage dealt by 30% for the duration.
Both of these would be roughly a 5% increase. (15% * (15s / 40s) = 5.6%) or (30% * (23s / 120s) = 5.7%)
These I feel are more interesting changes, but alternatively, they could simply buff several of our GCDs/oGCDs by 50-100 potency (as well as their DA effects, if applicable, to keep DA priority the same), or increase Darkside to 25%.
I think a possible solution to this is to have it return a % of Blood equal to the damage that was absorbed even if it didn't pop. But honestly, I like trade-offs, and the existing TBN mechanics are acceptable to me. People are complaining a lot about the job's low risk/reward and I feel like the risk of TBN not popping is an acceptable instance of that kind of risk/reward playstyle that still is present with the job.