Um, guys, don't forget Black Flash is all about TIMING, not about speed. When using Black Flash, the speed of the attack isn't altered.
Also, didn't the dude who focused all his cursed energy in the stomach to tank a BF see Mahito using Black Flash beforehand? And isn't it the case that every time Black Flash is used, the user is in a flow-like state? It could be the case that his instincts merely realized that such state had been achieved and therefore there was a decent chance of a Black Flash coming, and he kinda "read" a Black Flash coming.
Both Yuuji and Mahito are very proficient at landing black flashes with decent frequency, albeit Yuuji more so, so this wouldn't be unimaginable by any means.
Anyways, given that not a single feat even from Top Tiers is close to the level of millionth-of-a-second reaction speed, I'm highly skeptical of the idea that a mere High Tier like a first grade sorcerer would have millionth-of-a-second reaction speed. We would literally be scaling Gojou Satoru's speed directly from "being faster than Yuuji", goddamnit!
Also, I do think we should take Black Flashes into consideration in VS threads, but in the same way in which we often do...
You know how a character having a 99% probability of defeating another makes it not a stomp? If a Jujutsu Kaisen character cannot damage their opponent unless via BF, but said character is indeed capable of using BF, then the fight shouldn't be a stomp.
Also, if a character cannot one-shot their opponent unless with Black Flash and should not a single BF happen the fight is perfectly balanced to the point of being almost inconclusive, BF can be used as an argument. I'm in favor of using BF as a stomp-prevention and tie-breaking argument.
Black Flash should never be used as a justification for a stomp such as in "stomps via Black Flash" because no character can Black Flash consistently. However, if such a character does appear, then we can use Black Flash as a justification for AP stomping.