I'm not entirely sure as I haven't read it specifically. But generally in Naruto fanfictions, it could involve new adventures of Naruto with a special black blade, perhaps facing new enemies or exploring uncharted areas of the Naruto world.
It could be about Naruto discovering a hidden blade - using technique and becoming a master at wielding it in battles against new and powerful enemies.
One interesting plotline could be the origin of the black blade. Maybe it's a weapon created by an ancient shinobi clan with a special power. For example, it could be a blade that absorbs the chakra of those it cuts, making the wielder stronger.
Naruto is likely to be one of the main characters since it's in the title. There might also be characters like Sasuke or Sakura who could be involved in the story related to the black blade.
One idea could be a crossover where Naruto characters get access to special blades with unique powers. For example, Naruto could get a blade that amplifies his chakra and allows him to perform new jutsu. Another could be a story where a hidden blade is discovered in the Naruto world that has the power to either save or destroy it, and the characters have to fight over its possession.
Naruto himself is often a popular character in these fanfictions. He might be the one who wields the black blade and uses it to fight against powerful enemies, bringing his usual determination and spirit to the story.
Perhaps Naruto is transported to the world of Unlimited Blade Works and has to learn to use its powers to get back home. Along the way, he could meet characters from that world and form new alliances. He might also bring some of the concepts from Unlimited Blade Works back to his own world and revolutionize the way ninjas fight.
Perhaps a plot where a blade master is secretly an ancient shinobi from a forgotten clan in the Naruto world. The character could hold the key to unlocking a powerful jutsu that is related to blade manipulation. As the story unfolds, they must fight against those who want to use this power for evil while also rediscovering their true heritage.
Another plot could involve a character from Blade of the Immortal being transported to the Naruto universe. This character could be an enemy at first, but then as they learn about Naruto's world and his ideals, they become an ally. They could teach Naruto some unique fighting techniques with the Blade of the Immortal.
Another interesting plot would be if the Executioner's Blade had a mind of its own. It could choose its wielder, and Naruto has to prove himself worthy to it. This could lead to a journey of self - discovery for Naruto as he tries to understand what the blade wants from him and how he can use it for good.
Perhaps in this fanfiction, Naruto enters the Unlimited Blade Works realm and has to face his own inner demons represented as clones within that world. This would be a great way to explore his character deeper as he battles his fears and insecurities.