There are changes I feel would make "TBN" "The Blackest Night" stand strong along with Heart of Corundum, Bloodwhetting, and Holy Sheltron. It feel that TBN has lost its potency as a Mitigation since Endwalker came out and made every other tank feel better mitigation wise in every way. My solution is simple, there's a couple minor changes that I feel would make DRK feel better both from the DRK's perspective and the healers. Right now there are fundamental issues with TBN.
First change should be how Dark Arts work. I like the system of using MP earlier into the fight and saving the Dark Arts for 1/2 minute burst windows, it makes you feel smart when you do it right, but when you miss-time TBN it doesn't feel like "High Risk High Reward," it feels like "Low Risk, High Frustration." To fix this feeling you should be able to stack 2 charges of Dark Arts. Right now if you already have a TBN proc, but 2 minutes is coming, then suddenly hitting TBN feels like a complete waste of 3000 MP. It would make sense if TBN was a 60 second cooldown but its 15 seconds, which is wasted on the mit as it stands.
Second, change how the TBN proc itself works. I feel that TBN should Always give you a stack of dark arts. You spent the MP, you shouldnt feel punished for mitigating. This change would give you that "free" edge with every use of TBN. This would make Dark Knights less scared to mitigate without feeling punished if they can only take a couple autos. Often times the better you are geared, the harder it is to actually fit TBN into a mitigation plan unless its a buster.
Third, there should be added mitigation to TBN itself. For this I would combine TBN and Dark Mind or Oblation. This would help get rid of button bloat in the job and make it a lot easier to mitigate during a 2 minute burst window. It feel that the devs are placing Tank Busters inside of burst windows, making the already busy DRK insufferable at times while you try to hit all 15 OGCDs if you really needs to Mit during two minute. Sometimes it feels even worse if its a Pot windows like the 6 minute burst of P8S part 1.
Fourth, to keep this balanced the cooldown on TBN should be better aligned with BW and HoC. TBN if these changes are made, should be on a 25 second cooldown.
To unlock this improved TBN, there should just be a trait at around 83-87 called "The Blackest Night Mastery"
Just to Summarize this is what the Tool Tips on TBN would look like with these changes.