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.
In this fanfiction, his black blood could symbolize his inner turmoil. Naruto has always faced a lot of hardships and darkness in his life. The black blood could be a physical representation of all the negative emotions like anger, hatred, and sadness that he has suppressed over the years, now seeping into his very essence.
Maybe he was poisoned. There are many villains in the Naruto world who use poison. If Naruto was poisoned during a fight or an ambush, it could lead to him coughing blood as his body tries to fight off the toxins. Just like how Sasuke was once poisoned and had to be treated.
Perhaps in this fanfiction, Naruto is in a situation where he has no other source of sustenance. He could be trapped in a desolate place with no food or water, and the blood becomes a last - resort option for survival. This would be a very dark and extreme take on the character, as it goes against his usual values.
One reason could be extreme emotional pain. Maybe he witnesses the death of a very close friend or loved one in the fanfiction world. This overwhelming grief could cause such a physical manifestation as crying blood.
One reason could be the extreme pressure of being Hokage. He has so many responsibilities, like protecting the village from external threats and managing internal affairs. It might all become too much for him to handle, leading to a breakdown.
A possible reason could be for self - expression. Naruto is a very expressive character, and tattoos could be another way for him to show who he is. It could also be a way to distinguish himself from other ninjas.
In some fanfictions, it could be related to a curse or a dark magic spell. There could be a hidden plot where an ancient curse is placed on Naruto, and as part of its manifestation, a hole appears in his chest, which could lead to all sorts of consequences for his health and his ability to use chakra.
It could be due to a genetic mutation in the Naruto universe. In some fanfictions, there might be a hidden genetic factor in his family line that gets activated. For example, perhaps Minato or Kushina had a recessive gene for white hair that Naruto inherits and it suddenly shows up under certain circumstances, like extreme stress or a coming - of - age moment in his ninja journey.
Maybe they could be a result of some forbidden jutsu gone wrong. For example, Naruto might have been experimenting with a new form of chakra manipulation that accidentally summoned some dark energy which manifested as black wings.
Maybe he discovers some dark secrets in the Blood Prison. These secrets could be about the corruption of the ninja world or some hidden evils related to powerful people. This new knowledge might make him lose faith in the system he once believed in, leading to his transformation into a darker version of himself.
Yes, it is possible in fanfiction. In the Naruto universe, Susanoo is typically associated with the Uchiha clan. For Naruto to have Susanoo in fanfiction, one idea could be that through some kind of chakra - related experiment or perhaps a connection with an Uchiha's spirit. It could make for an extremely powerful Naruto. He might use Susanoo's strength combined with his own Sage Mode or Nine - Tails chakra, creating a whole new level of combat abilities.