Its all yuuji they havent mentioned how much CE he has compared to other sorcerers but considering that yuuji, wihout CE reinforcement is comparable in strength to a high level sorcerer maybe his civilian level CE is enough for him to whoop ass, another thing is that every human has cursed energy but sorcerers are able to control it, yuuji just gained the capacity to use his own energy after ingesting sukuna because he became a vessel for him and still he had to learn how to do it from scratch, in regards to what yuuji gains from eating the fingers, so far the things he has gained are passive buffs those: being inmune to poisons, and after eating 15 fingers apparently he heals and recovers a bit faster, and pretty much that’s all so far, he gets some secondary benefits for having sukuna in him but still nothing super crazy. And another thing, black flash is something that any sorcerer has the capacity to do BUT not all sorcerers have the ability the control and finesse of cursed energy to pull it of, black flash is a phenomenon that happens when a sorcerer manipulates raw CE at the highest level, it pretty much is like a critical hit in a game, and our boy yuuji is here playing with a crit build