Black Flash requires characters to apply cursed energy less than a microsecond from the impact. This is a timeframe issue.
Yuji before applied the energy then struck with it, but then later he started to apply cursed energy right when it struck.
He needed to do it within 0.000001 seconds of said impact. The issue is that he couldn't do it at will. No one can.
Databook elaborates.
"Due to its
circumstance there is no sorcerer able to exert it at will"
Because of the circumstance that causes it to happen, the small timeframe, nobody can do it at will.
For a MHS character, this would be easy. The average arm length is about 0.76 meters long. Punching at around Mach 2000 and you'd have a large distance to utilize in order to push CE through your body to hit it at the "last moment".
Yuji would end up accomplishing a Black Flash every time he struck if he moved that fast. Same for everybody else.
The entire concept of sorcerers using Black Flash is skill and luck
because they aren't fast enough to do so.
A person could theoretically tag a speed of sound attack irl if they were skilled enough to time it, and it requires skill because they're naturally too slow to tag it.
Black Flash does not
need skill. It
needs the mechanic of imposing CE in a certain timeframe. For sorcerers, that timeframe is insanely high, which is why it's a random timeframe that needs focus and concentration.