Black Flash is a distortion in space that occurs when cursed energy is applied with 0.000001 seconds of a physical hit. When a sorcerer is able to achieve this, their cursed energy flashes black, and the destructive power of their strike is equal to a normal hit to the power of 2.5.
This technique requires incredible concentration and no jujutsu sorcerer is capable of using it at will, not even Satoru Gojo. However, the actual achievement of Black Flash suggests that the user has large potential and skill as it was stated that "the sparks of black do not choose who to bless", eliminating any chance of it being random. Sorcerers who managed to unleash Black Flash have an immeasurably higher understanding of the essence of cursed energy. Sorcerers who succeed in landing a Black Flash will temporarily enter a state of increased cursed energy awareness, where the mental process of manipulating cursed energy becomes as simple as breathing for them.
Nanami believes that using Black Flash consecutive times isn't anything to celebrate. After using the technique more than once, doing so repeatedly is not as difficult as initially releasing Black Flash. Nanami compares using Black Flash to being in a "zone" of focus similar to what professional athletes experience while in a game, feeling as if they're the center of everything. Nanami's record was four consecutive uses of Black Flash but accredits this to getting lucky.
During the Goodwill Event, Aoi Todo was able to teach Yuji how to harness his cursed energy by combining it with his superhuman speed and strength. By explaining how to make his cursed energy flow properly and pitting Yuji against Hanami, Aoi was able to help Yuji unleash the ability. The attack was able to do great damage to Hanami just from one strike. By landing multiple Black Flash attacks while fighting alongside Aoi, Yuji was able to overwhelm the special grade curse, equaling Kento Nanami's record of four consecutive Black Flash strikes. He used it once again against Eso & Kechizu, and against Mahito during the Shibuya Incident. Altogether, he has used it nine times.
After emotionally breaking Yuji Itadori by near fatally injuring Nobara Kugisaki, Mahito landed a Black Flash strike on the unprepared Yuji, launching the boy across Shibuya Station and overwhelming him with numerous attacks. He once again used it against Aoi Todo after weakening him with Idle Transfiguration.
During the fight Yuji and Aoi had against Mahito in Shibuya, he managed to perform a Black Flash by kicking Mahito, not wanting to be left behind his "brother's" growth. However, this was unable to do any lasting damage to the cursed spirit, due to his soul being unaffected by the attack.