Most of the Stormblood changes were the result of removing the MP drain from Darkside. Prior to this, MP generation was in a natural state of balance between Darkside and Syphon. You had a relatively slow rise in MP over time, but you also needed to have some MP in reserve for abilities like Dark Passenger and Carve and Spit. Dark Arts was still important, but it tended to come into play during periods of higher MP generation, like Blood Weapon and Blood Price.

Removing the MP drain from Darkside created several issues. First, the MP generation from Syphon was unopposed. Second, you have the addition of the new Delirium which is essentially massive MP dumps. Also, with the removal of Dark Passenger as a viable single target ability, you suddenly have an extra 2 DA worth of MP every minute.

The devs' solution to keep up with this MP excess was to replace the MP drain of Darkside with Dark Arts uses. But in order to keep up with demand, you now had to be able to use it on Syphon as well as Souleater. So if you want to use Dark Arts on an oGCD, it's a forced double weave. If you want to use any regular oGCD, it's also probably a double weave, because you need to constantly use Dark Arts to avoid capping.

I see people occasionally mention that Dark Arts gives you flexibility to "burst when you want." This shows a complete and utter lack of understanding of how the job works. Your MP bar is not big enough relative to the MP cost of Dark Arts to store up extra uses for a rainy day. You use it, or lose it. Burst is not burst. You don't dps harder. You dps longer.

All in all, it was a series of incredibly bad decisions.

If you want to fix it, the first thing that you need to look at is MP generation. Right now, all our MP generation happens on Syphon as one big block. Why? On WAR, they had the common sense to space it out. It's not like all your abilities generate no gauge until you hit Maim, at which point you have 30 extra gauge. Spread it out so that everything generates a small amount of MP. That way, you avoid having these massive MP dumps.

Delirium is another problem here. I'm not sure why the blood cost is there at all. IR had a much smaller resource cost to use at the start of this expansion and it was removed, despite it being a thousand-fold more powerful. But if your goal is to build off of Blood Weapon, it's done in completely the wrong way. What are we going to do with all this MP? We're not going to spend it faster. It's just going to take longer to burn through.

Instead of focusing on Delirium's MP generation, they should have focused on speed. There's a natural limit at which speed boosts are effective, of course. The faster your GCD, the less room you have for double weaving. But there are other ways to do "fast". Have actions hit twice. Shorten the recasts of oGCDs such that they come up very quickly while the window is active. Make it actually feel like it has an impact, instead of being just a longer Blood Weapon.

On the MP expenditure side of things, you need more ways for people to spend it. Carve and Spit should have the MP cost built in. Dark Passenger should be something that you actually want to use in single target.

And as for Dark Arts itself? In a perfectly balanced system, you'd have enough resource spending oGCDs such that Dark Arts didn't need to be constantly used. But I suppose that the easiest solution is to have it last for the full 10 seconds, boost any abilities used while it is active, and just replace MP generation with blood generation. Use Dark Arts, your MP generation stops, and your Blood generation starts.

Also, this bit about TBN being equivalent to self-healing is nonsense. Pick an older fight to solo and try to "self-sustain" yourself using just TBN. There's no way of recovering lost health, so if the fight drags on for long enough, you're going to die. It's like saying that Hallowed represents an infinite amount of self-healing.