From my own analysis on the 2. TBN could be a marginal dps gain or marginal dps loss. There is no complete break even scenario.
It depends really. The simplest way to look at it would be that for every TBN you use, your adding a bloodspiller into your maximum GCD count before boss dies but that GCD count is finite so you have to remove the last combo gcd you would have otherwise used for each bloodspiller added this way.
Dark arts + souleater is 440 potency
Dark arts + syphon strike is 390 potency but every second syphon strike generates another instant 140 potency, You could say syphon strike actually does 320 potency.
Hard slash is only 150 potency.
Say you used TBN and clipped a souleater off your maximum gcd count at the end of a fight. You lost 140 potency from the dark arts and 300 potency from the souleater. it was 400 bloodspiller potency vs 440 souleater+dark arts potency.
Say you used TBN and clipped a odd numbered syphon strike off your maximum gcd count at the end of a fight. you lost 140 potency from dark arts and 250 potency from the syphon strike. It was 400 bloodspiller potency vs 390 syphon strike potency.
Say you used TBN and clipped a even numbered syphon strike off your maximum gcd count at the end of a fight. You lost 280 potency worth of dark arts and 250 potency from syphon strike. It was 400 vs 530 potency.
Say you used TBN and clipped a hard slash off your maximum gcd count at end of fight, you lost 140 potency from dark arts and 150 potency from hard slash. 400 blood spiller potency vs 290.
Wow TBN looks like its actually increasing my damage as long as hard slash is clipped and/or an odd amount of syphon strikes! How do you even plan something like that.
Say you used TBN 3 times in a fight just to gaurentee you clip a hard slash.
You clipped off 1 hard slash 1 syphon strike and 1 souleater for a total of 150+250+300=700 You clipped 3 dark arts for 420 and the total is 1120
3 TBN's translates to 3 bloodspillers so 1200 potency
You gained 80 potency, hey it works! But it gets tricky.
We cannot plan the end of a fight. Say you clipped the even numbered syphon strike. That last syphon strike you clipped would have given you another dark arts to use so in reality you clipped 4 dark arts and if you had been using dark arts instead of TBN you would have had another 140 on top of that 1120 putting you at 1260 potency vs 1200. It gets worse if you ended the fight on 40 blood and clipped a souleater, instead of clipping a syphon strike or a hardslash you could actually clip a dark arts and a natural bloodspiller for a bloodspiller and 40 blood remaining. The easiest way to show this would be like this: Say you ended your gcd count on a hardslash having not used 3 TBNS. If you had used 3 TBNs 3 backwards from a hardslash>Souleater>Syphon strike, this resulted from 3 TBN's used, those 3 combo gcds are lost in that order.
Here's a possible example: Say you used TBN 3 times.
Say you clipped 1 hardslash, 1 syphon strike, and 1 souleater, and 3 dark arts at the end of the fight you had half a dark arts MP and 40 blood. That last syphon strike would have given you another dark arts and that last souleater would have turned that last hardslash into a bloodspiller. So in reality you clipped 1 syphon strike, 1 souleater, 1 bloodspiller, and 4 dark arts. 250+300+400+(140*4)= 1510 potency vs 3 blood spillers at 1200 potency.
The reality: Is TBN always going to be a potency loss? No. If you can successfully clip only a hardslash off your total gcd count or an odd numbered syphon strike by all means go for it. If you end the fight clipping any number of souleaters that could otherwise result in a naturally generated bloodspiller with an extra GCD to use it, you've lost potency (At 2 TBNS and higher this becomes possible by simply clipping a souleater and a hardslash and 40 blood remaining at the end of a fight. Whether or not its worth worrying about if its a gain or a loss is up to the player. The loss or gain is miniscule in the grand scheme of a whole fight encounter potency wise but it doesn't hurt to know more about how the skill affects you.
I've thought about making a spreadsheet of Dark arts VS TBN use for gcd counts starting from 50 but it seems like more trouble than its worth. Maybe someone else can actually do an in depth look at dark arts VS TBN to find out exactly how and when each results in higher potency output but for now these are just general rules I've learned for myself. I'd rather assume TBN is a dps loss than risk it for a very marginal gain. If you are going to use TBN in a fight, I would recommend using it atleast 3 times to gaurentee clipping a hard slash at the very least. If it all goes smoothly and you clip a regular combo hardslash>syphonstrike>souleater without clipping an even numbered syphon or natural bloodspiller, you gain 80 potency. If you clipped the even syphon though you lost 60 potency. One could argue you need another GCD to be able to use that dark arts anyway but for the sake of that argument you could get 100 potency instantly from dark passenger instead of 140 from dark arts for a 20 potency loss instead of 80.
I basically revived this discussion because it seems like there are still different beliefs on this topic floating around as to whether or not TBN is a gain or a loss. Don't take anything I've put here as fact, merely my general thoughts and own analysis. While I don't hate how dark arts and TBN currently work, I'm not happy with them either.