One difficulty in moving abilities around is the job quests. The Blackest Night, in particular, is strongly tied in to the Dark Knight job lore at level 70. I think that max level job balance is a more urgent issue than the play experience at lower levels.
Now that I've had some time to mull it over, I think the problem is that we've gone from being a job with more sustained dps, through DoTs and a steady stream of oGCDs, to one that tries to be more burst orientated without the job mechanics to support it.
To illustrate:
WAR:
PLD:
DRK:
(These are just some examples; the first two are from the same run, but the third is a slightly longer one. Just treat this as purely for illustration purposes.)
WAR has the vast majority of their damage output concentrated in their Berserk windows, once per minute, with a bigger burst every second Berserk due to IR. This has really good buff synergy, especially with TA/Embolden. It's also much simpler to optimise this style of play in lower uptime fights, because the damage is all in one place. PLD tends towards less burst, but more consistent damage, and tends to be more competetive in higher uptime fights.
DRK isn't too bad as far as the opener goes, when everyone's buffs are synced up. The 40s recast on BW puts us out of phase with most raid buffs until the 120s mark, as far as DA potency dumps go.
I'm wondering what would be the impact of dropping BW down to a 30 second recast. You'd have higher overall buff uptime (albeit on a relatively weaker buff than Berserk/FoF/Req), but you'd also be able to sync up with every TA like PLD/WAR instead of every second one.
Bloodspiller
Another issue from a burst standpoint is that individual Bloodspillers aren't as much of a potency dump as DA tends to be. Part of this has to do with relatively high combo potencies, and part of it has to do with the value of the resources tied to each combo step. It's actually surprising how important Syphon Strike is; when you factor in the MP gain, it's worth only a few less potency than Souleater.
As a point of comparison, Bloodspiller is worth 300 potency when it replaces HS, 96 on SyS, and 100 on SE. If you look at FC on WAR, FC is worth 420 potency when replacing HS, 310 on Maim, 230 on SE, and 197 on SP before factoring in Berserk. So each individual hit has less of an impact.
I think that your options are either to increase the net value of Bloodspiller, or to make it come up more frequently. You can address the former by either raising the potency or adding in MP gain. I'm in favour of MP gain, in part because it removes the feelsbad sense that you could be gaining resources off of SyS instead. It also removes the question of whether you should use TBN because it may or may not be a dps loss. We already know that the new Quietus-TBN synergy feels fantastic. Why not bring it over to single target?
Off-Globals
I think the greatest impact to our relative dps comes from the loss of most of the HW oGCDs (Reprisal, Low Blow, and the DP nerf). I can see the reason behind it, with the increase in DA usage. That being said, the remaining oGCDs are in a weird place. DP doesn't have a place in single target, and in AoE DADP for 50% of your MP seems pretty steep.
DA C+S doesn't get discussed much, but it's a net 372 potency/min under Darkside vs. a net 468 potency/min for Upheaval (factoring in SE and the gauge cost) and 600 for Spirits Within. Also, double-weaving DA with C+S doesn't feel good, and there's really only one place in the combo cycle to single weave it. Actually, double weaving DA with any oGCD doesn't feel good, especially during BW, because of the animation length.
I think it probably makes more sense to just build in the resource costs at this point. Perhaps have C+S cost blood and restore MP, while having DP cost MP and restore blood (while actually setting DP's potency/cost so that it's actually usable in single target) to keep that sense of resource exchange going.
Delirium
I think that there's a couple of problems with Delirium. The biggest is that, between the upfront resource cost and the total effect, it doesn't offer that much additional potency to be worth the 2 minute recast. I also think that the idea of a buff extension also runs contrary to most raid buff windows, where the goal is to fit as much potency into a fixed time interval as possible. So while the 8 extra seconds gets you more resources over the total interval, it's not really going to get you extra resources within a 10 second TA, for example.
I feel like this skill needs a rework, and I think that the focus needs to be on intensity rather than duration. My first thought was something along the lines of having DA persist for multiple attacks (similar to DA's original bug effect at launch), but that would leave you without a way to properly dump MP. Some other possibilities would be to have DA potency effects increase under Delirium, or to have DA generate blood and blood usage generate MP. Either way, it's too finicky and offers too little net benefit for the cost at the moment.
I know that there's more issues defensively as well, but I wanted to touch on just the main offensive issues at present (assuming a fight where all three tanks have the freedom to optimise with minimal stance usage, of course).