It might be due to a genetic quirk from his family's hidden magical heritage. His parents or some distant relatives could have had a dormant gene related to eye color that got activated in a strange way. In the wizarding world, there are many things that are not fully understood about genetics and magic, and this could be an interesting exploration in the fanfiction.
One possible plotline could be that Harry got injured during a battle against a dark wizard, resulting in the loss of an eye. He then has to adapt to his new situation while still facing the challenges in the wizarding world. For example, he might struggle with his depth perception during duels.
Fanfiction usually stays within the fan community and doesn't directly affect Harry's story in the official books. So he has no reason to fight it.
One possible reason could be that he got seriously injured during a battle against the Dark Arts. Maybe a spell backfired and damaged his limbs or spine, requiring extensive physical therapy to regain his mobility.
In fanfiction, there are numerous interpretations for Harry having scars on his back. It could be a symbol of his past traumas. For example, he could have been bullied at Hogwarts in a more extreme way that left physical scars. Or it could be related to his family history. Maybe there was some dark family secret that involved a ritual which scarred his back. Additionally, it could be from a forbidden magic he tried to learn in order to defeat Voldemort once and for all, and the magic had unforeseen consequences on his body.
One possible storyline could be that Naruto somehow gains a Hyuuga eye through a unique experiment gone wrong. This new power could initially be difficult for him to control, as the Hyuuga eye has its own set of abilities like the Byakugan. He might struggle to adapt to the sudden change in his vision and the new skills it brings, such as seeing the chakra points clearly. This could lead to some comical moments as he accidentally activates the eye at inappropriate times, but also some intense battles where he slowly learns to master this new power and use it in combination with his existing Rasengan and other techniques.
His enemies would likely view it as a sign of weakness or something they can use against him. They could spread rumors that he's been cursed or is a danger to the wizarding community. On the other hand, his closest friends would stand by him no matter what. They would try to understand what happened and help him deal with any difficulties that arise from having such a distinct physical feature. It could also lead to some self - discovery for Harry as he sees how different people react to this change in him.
One possible reason could be some sort of dark curse or forbidden jutsu. Maybe Naruto was the victim of a malicious experiment that altered his blood composition. Another reason might be related to his connection with a dark entity. For example, if he has some link to an ancient evil, it could manifest in his blood turning black.
It could be a result of a magical spell gone wrong. Maybe he was experimenting with some powerful magic, and it had an unexpected side effect on his appearance, turning his hair long and white.
One reason could be that Harry is under an extreme amount of stress. He has so many burdens on his shoulders in the wizarding world, like fighting Voldemort. This stress could manifest physically as a seizure, especially if he is in a tense situation in front of Snape, who is often a source of anxiety for him. For example, if Snape is about to accuse him of something he didn't do while he is already worried about other threats to the wizarding world.
One possible reason could be for protection. Maybe in the fanfiction, there are dark forces after Harry, and giving him a different name is a way to hide his true identity from his enemies. It's like in the original story where his mother's love protected him, but here, a new name adds an extra layer of security.