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.
It could also be due to the influence of certain characters in the fanfic. There might be someone who has a strong dark chakra or a very negative ideology that slowly affects Naruto. For instance, if there is a powerful and evil inmate who constantly tries to persuade Naruto to embrace the dark side, over time, Naruto might be influenced.
One reason could be a major betrayal. For example, if his friends turned on him, it might shatter his idealistic view and push him towards violence. In fanfic, an author might explore this by having a false accusation against Naruto within the village, causing him to lose faith in the people he once trusted.
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 possible plot could be that Naruto is wrongly accused of a crime he didn't commit. Maybe he was framed by an old enemy who wants to see him suffer. While in prison, he tries to prove his innocence. He could use his ninja skills subtly to gather evidence within the prison walls, and also make some unlikely allies among the inmates who start to believe in his innocence as they get to know him better.
One possible reason could be a traumatic event. Maybe he witnessed something so horrible that it made him shut down his emotions to protect himself. For example, if he saw all his friends killed in front of him, it could be too much for him to bear and he just numbs himself.
In this type of fanfiction, the plot can be really diverse. It might start with Naruto being sent to the Blood Prison for some false accusations. Once there, he encounters various characters, some of whom are trying to kill him, while others might become his allies. The Blood Prison setting allows for a lot of exploration of Naruto's character in a darker context. It could also involve Naruto having to use his skills in new and creative ways to survive, and perhaps there's a build - up to a big showdown against the main villain or the corrupt forces running the prison.
Well, in such a fanfic, Naruto's child could be a key element in changing the situation in the Blood Prison. Since Naruto has a child, he might be more determined to break free from the prison's control. The child could also be a target for some evil forces in the prison who want to use the child's potential for their own purposes. And Naruto would do anything to prevent that, leading to intense battles and emotional moments.
Might be one of the powerful guards who has a grudge against Naruto for some reason. Maybe he believes Naruto is a threat to the order in the prison and tries to get rid of him secretly.
It could be that Naruto is sent to prison as part of a political move. There are some ninja clans within the village who are jealous of his popularity and influence. They conspire to get him out of the way by making it seem like he's a threat to the village's security. In prison, Naruto discovers a hidden conspiracy within the prison walls that is related to the outside political turmoil and he has to find a way to expose it while also getting himself released.
There are indeed alternatives. Some fanfics might explore the relationships between Naruto and other characters in the Blood Prison context. For example, they could depict how Naruto interacts with the guards or other prisoners in a way that promotes the values of the Naruto universe, such as perseverance, loyalty, and friendship. It's important to avoid the offensive ones and seek out the positive fan - created stories.