Yuji can objectively Black Flash at will
Sorry if i’m late to this lol but i just wanted to share my opinion and if anyone’s open to agree and disagree feel free to comment!
First, I want to point out the distinct SFX for a regular CE punch and RCT. As you see in image 1 & 2, the SFX is being used specifically for RTC, as Gojo uses it against Sukuna, and Yuta uses it against Kurourushi. The SFX in image 3 & 4 are used for regular punches with CE for reference.
The 5th image is Yuji throwing a punch with the exact same SFX as the RCT panels.
Now that we can OBJECTIVELY agree that Yuji is using RCT in his punch, the question is why? To heal the Simurian?
Now, the 6th and 7th image shows Gojo explaining Black Flash. He specifies that factors like air humidity and temperature are relevant to what can make a Black Flash occur, since it’s fundamentally a distortion in space.
Yuji didn’t crack his finger, but actually broke it. And used the steam created from RCT to change the air temperature and humidity, fulfilling those specific prerequisites for Black Flash.
The other requirement is the 0.000001 seconds rule, which hopefully we can all agree Yuji had that down since 2018
Now for the narrative. Yuji says “you look pretty tough, you’ll be fine” and then continues with “you won’t die”. Why would he give such a warning for just a regular punch? Why not just punch him? Yuji's specific words in Japanese are "死にはしない" which entails a firm stance, like he’s saying “i’m sure your not gonna die” instead of “you’ll be fine i think”.
Yuji OBVIOUSLY knows he’s going to hit a black flash and actually warns him. First and only time in the entire series a black flash was used so casually and nonchalantly.
The series also is constantly rephrasing “no one can use black flash at will”, almost as if it’s a precursor to “except for ___ of course” I mean, the guy hit 8 (EIGHT) black flashes in a row against the strongest sorcerer in history.
To conclude, it is almost impossible to argue that Yuji Itadori cannot land a Black Flash "at will". While i don’t think it's something you can just activate like a technique; its more like he’s capable of optimising the chance for Black Flash to 100%. He threw one punch the whole module series, so technically he’s 100% (1/1) with his black flashes.
Especially in Gojo vs Sukuna, we see sorcerers break the rules or do the impossible all the time. It’s not ridiculous to say Yuji is one of those sorcerers.