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.
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.
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.
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.
Well, he might find it in some mysterious place like an ancient temple. Maybe it was left there by a powerful being from another dimension.
Sasuke could be one to help. Given their history and Sasuke's intelligence, he might contribute some knowledge about energy manipulation which could be useful for the lightsaber's power source.
The black lightsaber has a unique and interesting story. It was created by Tarre Vizsla, the first Mandalorian to become a Jedi. It's a symbol of power and has passed through different hands over time, often being associated with important events in the Star Wars universe.
Well, in such a fanfiction, Naruto might use his chakra in a unique way to power the lightsaber. Maybe he'd combine his ninja skills with the new weapon. For example, he could use his speed to make quick slashes with the lightsaber, like his fast kunai attacks.
In the fanfiction, the double lightsaber might be a gift from an unknown entity. Naruto, being the curious and adventurous type, would start training with it. He'd have to learn how to handle the two blades simultaneously, which could be a whole new challenge. This could lead to new forms of combat that blend Naruto's existing fighting styles with the lightsaber techniques.