Sasuke would likely be intrigued. He has his own dark path to some extent. He might see Dark Naruto's departure as a bold move and wonder if he could have the same courage. At the same time, he might also be a bit envious that Naruto is able to break free in such a way while he is still tied to the village in some ways.
Sakura would be heartbroken. She would probably blame herself for not being able to protect Naruto. She might also start to train harder to fill the void left by Naruto.
The other genin competitors would have a variety of reactions. Some might laugh at Naruto, thinking he's being foolish. However, others might be inspired by his boldness. The proctors of the Chunin Exams could be either strict and try to stop him if it violates any rules, or they could be lenient and see it as a creative form of self - expression. And Naruto's friends from the village would likely cheer him on, seeing it as just another one of Naruto's unique antics.
Naruto would surely be one of the main characters as it's about his experience in the Chunin Exams, even if it's a darker version. Sasuke might also be a key character as he is an important part of Naruto's story during the exams.
Well, it could be about Naruto during the Chunin Exams but with a darker twist. Maybe it focuses on Naruto having to face more sinister opponents or dealing with some kind of internal struggle that's much darker than in the original story during the exams.
Grimmjow could be seen as a new kind of threat. The Chunin Exams are a test of a ninja's skills, and Grimmjow has skills that are completely different from the ninja arts. His spiritual power could be something that the ninja have never encountered before. So, he could either be a great ally if someone can convince him to work with them, or a terrifying enemy. He could also disrupt the normal flow of the exams as he doesn't care about the rules and regulations set for the ninja.
In the fanfic, Naruto might use intense physical training like running up mountains. This helps build his stamina which is crucial for the exams.
Perhaps in the fanfic, Dark Naruto realizes that his presence is causing unrest in the village after the Chunin Exams. He doesn't want to be a source of trouble for those he cares about, like his friends or even his sensei. So he chooses to leave to protect them. He might embark on a journey to learn more about his dark side and how to control it away from the village.
Even Kakashi could play a role. He might oversee Naruto's training and correct his mistakes, ensuring Naruto is well - prepared for the Chunin Exams.
It varies a lot depending on the fanfic author's imagination. Sometimes, they might add new challenges or twist the existing ones.
The other genin would be in awe. They would see Naruto as an almost unbeatable force. Some might be scared, like those who are not used to seeing such a powerful display. For example, the weaker genin might feel completely outclassed.