After getting dark knight 60 and playing it for a while

[Xenosan]

For single target, if MP management is a factor, unbuffed Passenger (+150 oGCD) with unbuffed Souleater (150 + 260 > 400) or Delirium (150 + 280 > 400) to get a 'better' Potency than strictly DA'd Souleater - but at half the MP cost.

Pretty much every 30 seconds you're getting a buffed SE combo for half it's cost (tho less self healing). The Passenger DA buff is only really worth it for trash mobs multiple targets. In single target, the 884 cost of using Passenger added to the 1769 MP cost of Dark Arts means that 250 + 260/280 is coming at 1.5x the MP cost of buffed SE (400), and unless there's MP needed to be wasted - it's better to put the .5x MP into a second buffed SE. With the Dark Arts 5 second recast, it can be difficult to load a DA buffed Passenger and DA'd SE in the same combo

Snipped

Your WS/CD analysis are more or less the same as a lot of others. I agree 100% with this:

Dark Arts: MP cost is steep, but that's okay, it makes you be more aware when playing Dark knight. As you can see above, I think it should be more active in Dark knight's kit. It's the main gimmick of the job, and I feel like it isn't used enough.

^AST is in the same situation.

Dark Arts & Card Drawing are the job-identifying actions for each class. BUT, instead of these abilities actually adding to and enhancing a solid core, CARDs and DARKNESS serve to allow a PLD-core (inferior) DRK to play pretend at (inferior) WAR, and a WHM-core (inferior) AST to play pretend at (inferior) SCH.

--
Said that in a general sense Tranquil