The weapon stance system proposed is simple and wouldn't matter in the slightest, all things considered.

Everyone loses on using their threat combos. Dark isn't unique. The discrepancy between the three threat combos isn't even that large to make the term 'worst' have any weight and I'm pretty sure that goes to Paladin.

Syphon might as well not exist with Spinning Slash gaining the MP return (which undermines the proposed stances further). No, one of them doing +30 potency but having the same basic function -while one also does threat- doesn't warrant having two buttons. That's like literally a case of where they'd cut abilities in the next set.

Blood Weapon would be 10 seconds, not 13. Removing Delirium from the equation means you can't use Delirium's duration to factor in uptime on the switch.
Blood Price should simply restore Blood Gauge proportional to the damage taken. That alone makes it worthwhile to use in Boss Scenarios and is a side bonus to AoE tanking in dungeons.

Dark Arts being refunded just to act as a button consolidation method is really awkward.

Carve and Spit: There's no reason for it to have the MP gain. Given Darks have been asking for a high HP heal, Carve and Spit is the perfect place to put that, perhaps as a 200% HP drain.

Dark Passenger is just another button you spam now. (And like, I feel I need to say this, is that I'd want -more- OGCDs to require some thought put into them, not less). It could be 140/280 and it'd still be a braindead spam because it's twice as efficient as Dark Arts in the base form, and then you can dark arts it again to double its effectiveness. 150% of the DA cost for 200% of the power.

Dark Mind: It is objectively good. I guess even Dark Knights need Divine Veil though (Because that's basically what this boils down to.)
Shadow wall: I want shadowskin back too, but I wouldn't do it like this.
Living Dead: Why, when you have Shadow Wall? This kind of highlights the issues I have with how you're setting up the mitigation tools. "I want this back. Oh but wait, if I do that I won't have the other one. Well I'll have this one swap to that. But wait, if I do that..."

Blackest Night: But...it'd be DPS loss at 1 (2.4k for 40), a DPS loss at 2 (because it'd be 3k for 50 blood) and even larger DPS losses higher than that. You can just make the ability cost Blood and refund a greater amount of blood and that solves the 'DPS loss' issue. Despair only serves to punish a Dark Knight even more for needing the shielding.