Hi, Junko from Japan here! Black flash says KOKUSEN in the original Japanese word. What does it mean? It’s “black flash” as it is! All right, I’ll explain more based on the Kanji characters.
Here’s the original Kanji writing for KOKUSEN. The meaning of each Kanji symbol is “black” and “flash.”

KOKU = black
SEN = flash
The Meaning of KOKU
The KOKU symbol means black. This is the same letter as the one for FUSHIGURO’s GURO. In Jujutsu Kaisen, the cursed energy gives black light when Black Flash is succeeded. This KOKU Kanji shows that color.
The top part of the letter stands for “soot and smoke,” and the four dots on the bottom portrays “fire.” The whole character describes the process of making SUMI, black ink for calligraphy.
The Meaning of SEN
This SEN symbol means to “flash” or ” to have an inspiration.” It expresses that something shows up instantly in space or in your mind. I think you’ll see this Kanji in the names of special moves of Japanese anime and manga since this is a good character to describe a sword slash.
The outer part of the symbol depicts a “gate,” and the inner one is a “person.” The whole symbol tells the moment when a person passes through a gate.
What is KOKUSEN in Jujutsu Kaisen?
KOKUSEN is a critical attack that happens when the physical attack and the flow of one’s cursed energy come at almost the same time. It’s a very powerful attack while Jujutsu sorcerers can’t do it on purpose.
It’s said that those who succeeded in triggering KOKUSEN can get close to the core of cursed energy. That means the sorcerers become to be able to control their cursed energy more freely and mightily. In a nutshell, a Jujutsu sorcerer who has experienced KOKUSEN can be stronger.
Here’s the list of characters who can use Kokusen in Jujutsu Kaisen. I’ve explained the name meaning of each character in Japanese too. Check the article of your favorite character!
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