I think SE needs to simply pick a couple of spots to buff and focus those down; right now it seems like they can't make up their minds. Every buff they give us makes it harder for us to burst in any real capacity - they take turns buffing our nuke abilities and then turning around and buffing our generic combos so that the difference is nigh-nonexistent. Its almost as if the guy that designed 3.x DRK, 4.x DRK, and the 4.x buffs/adjustments are all 3 totally different people that don't talk or even commute to the same building every day.
I frankly could have done without the ridiculous buffs to our combo potencies and would rather have seen a more satisfying buff to Bloodspiller and Delirium like the ones you have stated here. If they want us to not have burst but be more about sustain we just become a poor-man's PLD.
Here's some concrete adjustments I'd suggest based on your statements:
Bloodspiller: Delivers an attack with a potency of 470, 610 while under the effect of Dark Arts. Ignores the Grit Damage penalty. Additional effect: Restores MP 1440 MP. Blood Gauge cost: 50
Carve and Spit: Delivers an attack with a potency of 450. Restores 960 MP. Blood Gauge cost: 20
Dark Passenger: Delivers an attack with a potency of 200. Restores 10 Blood Gauge. MP cost: 1800
Delirium: Restores 2400 MP and increases the damage dealt of Dark Arts augmented abilities by 40%, while increasing the Blood returns of Blood Price and Blood Weapon by 100%. Blood Gauge cost: 50. Cooldown reduced to 80s.
Dark Arts: Generates 10 Blood per cast.
Under these changes, Bloodspiller and Carve and Spit would restore 3x and 2x Blood Weapon procs worth of mana, respectively. C&S would have a minor gauge cost to offset the fact that it no longer require's Dark Arts. Dark Passenger would function the opposite way, being double its current potency for 1800 mana, making it mana efficient next to DA, but also restoring a decent amount of Blood Gauge. Dark Arts would have Blood generation built into it, and Delirium would no longer extend BP/BW but instead increase their Blood returns and increase the damage dealt of DA'ed abilities by 40%, while keeping the initial mana return and Blood Gauge cost the same. I would also have its cooldown cut so that it could line up with every other Blood Weapon and not every 3rd, giving it parity with things like Zerk/IR and GB-RA/FoF which also line up every other cast instead of every 3rd.