Perhaps it's given to him by a mysterious figure. This figure could be from another dimension or a very old ninja who has been in hiding for a long time. They recognize Naruto's potential and power, and decide that he is the one worthy of wielding the black lightsaber. This could be part of a bigger plan to help Naruto defeat a new and powerful enemy that neither the ninja world nor the normal means of combat can handle.
Another possibility is that the black lightsaber has some sort of special power or property that ties into the Naruto universe's existing powers. For instance, it could be attuned to chakra in some way. Maybe it can be powered by chakra instead of the typical energy source in the Star Wars universe. This would create an interesting dynamic as Naruto characters would have to learn how to manage this new form of power while still using their own ninja abilities.
Naruto's fighting style would likely incorporate more acrobatic flips and spins to take advantage of the lightsaber's reach. He'd also use his shadow clone jutsu in new ways, like having clones surround an enemy while he attacks from above with the lightsaber.
He could find it during one of his missions. Maybe he's exploring an ancient ruin and stumbles upon the lightsaber. It could be in a chest or just lying there, and Naruto, being curious, picks it up.
If Naruto wielded a black lightsaber in fanfiction, he would likely be an even more formidable opponent. His already impressive combat skills combined with the power of the lightsaber would make him a force to be reckoned with. He could use his speed and agility to quickly strike with the lightsaber, and his strategic thinking would allow him to use it effectively in battles.
Well, perhaps Naruto could use the lightsaber in his ninja battles. He might combine his chakra control with the unique abilities of the lightsaber. For example, he could infuse his chakra into the lightsaber to make it more powerful or use his ninja skills to perform acrobatic maneuvers while wielding the lightsaber.
He might gain the ability to cut through almost anything. The black lightsaber, being a powerful weapon, could enhance Naruto's already formidable combat skills. He could use it to slice through strong jutsu defenses with ease, something that normal kunai or shuriken couldn't do.
One idea could be that Naruto discovers the lightsaber in a hidden ninja temple that has some sort of inter - dimensional portal. He then has to learn how to use it without any prior knowledge of lightsabers. It could be a comical journey as he tries to figure out how to hold it and not accidentally cut himself or his friends.
One idea could be that Naruto discovers the lightsaber in a hidden ancient temple during one of his adventures. He initially has trouble controlling it due to its unique dual - bladed nature, but with his determination and quick learning skills, he soon masters it and uses it to fend off powerful enemies. This new weapon gives him an edge in battles and also makes his combat style more diverse.