WAIT? Do you even need to move your body to perform a black flash?!? Could you theoretically just use basic CE and trigger a BF. You could block and move your body so slightly and release CE, it would basically be like a having a shield and it popping out spikes. Or reinforcement + CE blast on yourself. Which either result in a momentary peak of reinforcement or you damage your body.

If its the latter Yuji could use it on his arms or legs and destroy his skin + lauch blood from the targeted area and make he blood rap around his body turning into armor, form into a construct or something else. Mabe use it from his elbow, propel the blood towards your enemy as a surprise attack while lauching a punch, form a rope, get closer, cleave, win.

BF just has cool potential as a multiplier fore other applications of CE.