It depends on your perspective. Some parts of the Pokemon manga have elements that could be seen as dark, like serious conflicts or character struggles.
One idea could be that Serena's 'cheating' might be a misunderstanding. Maybe she was seen with another trainer in a compromising position, but in reality, they were just discussing a very important Pokemon training technique that Ash didn't know about at the time.
The 'pokemon dark ash story' could be a story where Ash's character is developed in a darker light. It might start with something bad happening to his Pokemon or his friends, which sets him on a path of revenge or a more desperate journey. He could encounter new types of Pokemon that are exclusive to this darker version of the story. And the story might have a different kind of ending, not the typical happy - go - lucky one. Maybe Ash has to sacrifice something important to achieve his goals in this darker narrative. It could also show how the Pokemon world itself changes in this darker scenario, with different rules and power dynamics.
Well, it could be that there are anti - hero type characters. For instance, a trainer who was once a good guy but got corrupted by power or some tragic event. This character might have Pokemon that are also a bit more menacing in appearance or behavior. There could also be a mysterious figure who guides the main character through the dark aspects of the Pokemon world, this person might have their own hidden agenda. Another possible main character could be a Pokemon itself that has some special ability that makes it a key part of the dark stories, perhaps it can communicate with humans in a different way or has the power to change the environment around it in a strange way.
The Pokemon manga has its moments of darkness. It could be in the form of intense rivalries, high-stakes battles, or even emotional turmoil for the characters. However, it still maintains that Pokemon charm throughout.
For Pikachu, it might be very confused. Pikachu is very close to Ash and would sense the change in Ash's mood. It could start to be more protective of Ash and even be a bit hostile towards Serena. This would add an interesting dynamic to the story as Pikachu is usually friendly.