It's possible that the black sword has a connection to some ancient prophecy. Naruto is often involved in things related to prophecies. The sword could be a key element in fulfilling a certain prophecy, and it was fated to come into his possession. For example, it might be the only weapon capable of defeating a new and powerful enemy that the prophecy foretells.
The sword could have the ability to cut through any chakra - based attacks. Since Naruto's world is full of ninjas using chakra, this would be a very useful ability. It would allow Naruto to easily defend against and counter his opponents' attacks.
Well, in some fanfictions, Naruto might obtain the overpowered sword through a mysterious inheritance. Maybe it was left by a powerful ninja from the past and Naruto discovers it in a hidden cave or something.
Well, in some fanfictions, Naruto might obtain the Master Sword through a strange portal that connects his world to the world where the Master Sword exists. Maybe he's on a mission and stumbles upon it by accident.
In some fanfictions, Naruto might obtain the dragon sword through a special quest. Maybe he stumbles upon an ancient temple during one of his missions where the sword is hidden as a powerful artifact, guarded by some kind of magic or traps. He overcomes these obstacles using his ninja skills and determination, and thus gets the dragon sword.
Well, in some fanfictions, Naruto might obtain the Sword of Tengu through a quest. Maybe he has to help a spirit or an ancient being in return for the sword.
Maybe it has the ability to absorb chakra. So when Naruto is in a fight against an opponent with a large amount of chakra, he can use the sword to drain some of that chakra, weakening the enemy while strengthening himself. The sword could then store that chakra for later use, like to boost Naruto's own attacks.
Sasuke could be a great wielder. His natural affinity for lightning - based jutsu and his determination would make him a formidable user of the Black Lightning Sword. He could integrate the sword's power into his existing combat style, creating new and powerful attacks.
The origin of the cursed sword could be from an ancient and evil clan. Maybe they used dark magic to create it, and it was lost over time until Naruto discovers it by chance. For example, it could have been buried in a forgotten tomb that Naruto stumbles upon during a mission.
Well, in some fanfictions, he might inherit it from an unknown ninja clan. Maybe his parents left it for him and it was hidden until he discovers his true heritage.