A quick recap and update on our MP game:
Grit costs 20% MP
Darkside costs 6% MP, provides an additional 5% damage compared to now, and no longer drains roughly 3-4% every 3 seconds.
Blood Price: Assumed to still be 5% in addition to +5 Blood Gauge for 15 seconds every 40 seconds, Grit only.
Blood Weapon: 5% (Up from 3.5%) in addition to +3 BG for 15 seconds every 40 seconds, Gritless only.
Syphon Strike: Roughly 12.7% on combo
Carve and Spit: Roughly 12.7% if not using Dark Arts. Use Dark Arts.
Darkside's changes are obviously a massive buff to Dark Knights overall making MP management far simpler for not needing to keep the drain in mind.
Individually Blood Price can be looked at as a nerf, however Blood Weapon's buff sets it to be, give or take, equal to what is currently using both skills at once in a single target situation.
For those willing to stance dance a bit while actively tanking, you'll find yourself with much more MP availible to you than before even with Grit's cost, if you don't dance your MP should remain more or less the same as it does currently.
There are now three big cost abilities, taking 25% MP each. Dark Arts, Dark Passenger, and The Blackest Night.
Dark Passenger, while nerfed due to this doubled cost, is still more cost effective single target than using Dark Arts on anything other than Carve and Spit or Grit-Bloodspiller. DA DP has been further nerfed at a loss for 10 Potency, the combined total of 50% MP makes using this far more risky than I'd like outside of situations where you NEED the Blind.
The Blackest Night provides a 5 second buff on a 15 second recast timer. This buff shields yourself for 20% of your HP or another player for 10% of your HP. When it expires, you gain 50 BG. This will most commonly be spent on Bloodspiller.
Bloodspiller is a 380 (520 DA) Potency Weaponskill that ignores Grit's damage reduction. This is rather oddly displayed by stating that their Grit Potency is 475 (650 DA).
This will generally put TBN into rotation rather regularly. Something like TBN, HS, SyS, SE, BS, HS, SyS, SE, repeat. Ultimately you can try looking at TBN into BS as filling the gap left by Scourge (500 per 30s) with a little extra Potency on top (760 per 30s).
This does however lock SyS into providing your MP for TBN into BS, leaving you leaning on BP/BW for the rest of your spells. The lack of Darkside drain on top of Blood Weapon's buff should, as the MT, leave you enough with MP for two DA C&S's and 7-8 other DA skills or individual DPs within every 2 minutes of a fight provided you're stance dancing. My assumption is that these uses will be spent on DA BS when in Grit, DP when out of Grit, and DA SE if neither situation is currently applicable.
Aside from BP/BW there are two other generators for BG. Souleater and Salted Earth giving 10 and 1 per hit.
In general all this means is that you'll have to work in a few more BS hits aside from those given to you by TBN, but you'll also want to make use of the new Delirium
Delirium is no longer a Weaponskill but instead an ability on a 120s recast time that costs 50 BG. It gives you 25% MP immediately and extends Blood Price and Blood Weapon by 8s and 16s. I'm not sure which one gets which time boost as the tooltip I have seen is obviously screwed up saying that Blood Weapon and Blood Weapon get those timers. Ideally this has BW getting the +16s to extend how long you have until you may need to dip back into Grit for BP, going from a 15s Grit 25s Gritless to 15s Grit to 31s Gritless.
Update: That Famitsu link below seems to indicate that it's Blood Weapon getting +8s and Blood Price getting +16s, meaning that we'll be looking at 31s in Grit and 15s out of Grit when using Delirium on Blood Price to maximize MP and BG gains. It would still almost certainly be a DPS gain to use it on Blood Weapon however, though until we get some solid numbers I could be wrong.
Salted Earth's BG gain becomes enough to pay for our new AoE Weaponskill on its own at 8 targets at the end of its duration.
Quietus, like Bloodspiller and Delirium, costs 50 BG and it hits for 160 (210 DA) Potency.
I don't see this one getting anywhere near as much use as BS simply because at the numbers needed to use it regularly you'll probably be wanting DA Abyssal Drain. If only we still had access to Bloodbath, poor Quietus an expansion too late.
As for what we lost for all of this
Reprisal as damage, Low Blow as damage, Scourge, 50 Unleash Potency, and effectively two slots due to the Role Skills
At present, Cross Class provides Dark Knights with Provoke, Convalescence, Awareness, Foresight, Bloodbath, Mercy Stroke, and a whole lot of trash.
Role Skills will provide us with Provoke, Convalescence, Awareness, Rampart, Anticipation, Reprisal, Shirk, and a bunch of trash.
Provoke and Convalescence I don't likely see ever leaving our bars. Rampart is Shadowskin and likely has a permanent slot too. Anticipation is our old Dark Dance minus the option to DA it and is likely fighting with Awareness for the "RNG damage smoothing buff" but probably wins out simply due to frequency outside of autocrit fights like Diablos.
We can simply look at this at this point as having three CC slots as opposed to five, and we're trying to figure out what to put in number three. Reprisal as MT and Shirk as OT or on swap fights.
Overall I see Low Blow, Interject, and Ultimatum as largely useless outside of gimmicks. Ranged and Melee are pretty heavily lacking in slots that improve their damage output, leaving the job of stuns and silences better left in their hands while we improve our defenses.
Updates:
Dark Knights are potentially bringing vulnerability debuffs to the party now.
On the matter of Bloodspiller as Scourge's replacement:
Sources:
My own poking at my yard's dummy
https://www.youtube.com/watch?v=8ktzoXqa178
https://www.youtube.com/watch?v=3MqlCEiT-04
https://www.youtube.com/watch?v=eYzrmh71OoA