I've seen calc's that put the cursed spirit from episode 1 at around 10^7 joules (9-B) for Kool Aid manning through the wall, so if we said current base Yuji upscales to that, then with black flash he'd be at ~10^17 joules (7-B). Conversely, if you scaled Yuji to ~Todo and use the Grade 1 ~tanks, then that would have Yuji around 10^8 joules (9-A), he'd jump to some 10^21 joules (High 6-C) with black flash.
For ***** and giggles, if Gojo's striking strength is 7-C, then his black flash would be 5-B. (<- not suggesting this but exponents do be wild)
Albeit, due to the inflated nature of these values, the likely hood of that ^ line of thinking being accepted is low, but nonetheless still cool to mull over.