Um, may I interject in a few points here?
1. While I agree that Black Flash being an exponential increase in power would be NONSENSE, we don't even have to think to know that. That's because there's no way to square someone's power level, for example. What is the square of a gram of TNT? Is it 4184*4184 Joules? Is it one times one? Is it 4.184*4.184, based on kilojoules? You cannot square amounts of energy without having the unit very well defined. Therefore, I believe the very idea of applying exponentiation to power level is wrong to begin with. We should entirely discard the idea that Black Flash could ever have a "to the power of" increase in the strength of the attack. Instead, I believe it would have made more sense if we considered Black Flash to be a critical hit that increases the strength of the attack by 2.5 TIMES compared to what it was before. This is also compatible with characters who could be damaged slightly without Black Flash being moderately damaged when a hit with it is landed, and characters who could be damaged mildly or normally without Black Flash taking heavy but non-lethal damage (like when Yuuji tanks a Black Flash from Mahito).
2. Why are you people still saying that whoever can use Black Flash is MHS+ reaction speed? Not only has it already been stated that NO ONE in the verse can use Black Flash consistently (i.e, it's always luck ON TOP of a flow-like state), I've already explained that a person's timing can surpass their reaction speed by many, many times over. Just look at the legendary Daigo parry in which a human being hit fourteen frame-perfect (one sixtieth of a second) inputs in a row just through having an amazing timing. If a human doesn't react in any less than around a tenth of a second (usually it's estimated around two tenths for most people's visual reaction speed...), this would be six to twelve times more precise/faster timing than reaction speed. For characters who cannot even land Black Flash consistently and are always totally lucking their way into landing one, we can safely assume that they may as well be dozens or even hundreds of times slower than MHS+.