I'd imagine they would argue the 'gain' is the mitigation you get from TBN itself. So what they seem to be saying is instead of paying 2400 for 140 potency, you instead pay 2400 for 20%/10% HP Shield AND 140(?) potency.

This seems to reflect on their view of stances and tenacity as well. :L

They're not technically wrong in that (a broken)TBN does give you 'more' then Dark Arts, it's just we don't value the mitigation as much as they do.

-Spitball idea-

Gain the effects of Dark Arts(10 seconds) and increase the Blood Gauge by 50 when full 20%(10%) is absorbed.