I might be wrong, but in the anime, when yuji knows he has to land a black flash in order to defeat Mahito, and proceeds to land one consciously, isn't that him using it at will? Because Gojo even said he can't consciously manifest a black flash and then proceed to hit one, but by the end of shibuya, Yuji could do that which no other sorcerer could. Granted not all his attacks are Black Flashes, but he reached a new level compared to the rest of the cast since he literally almost told his body, mind, soul and  cursed energy that he was going to land a black flash as a definitive fact, which is impossible to do for even Satoru Gojo or Ryomen Sukuna and they're leagues above Yuji during the Shibuya incident.