Well, Naruto might use the lightsaber in a very unique way. He could combine his chakra control with the power of the lightsaber. For example, he could infuse his chakra into the lightsaber to make it stronger or to give it new abilities. Maybe he would create some kind of new jutsu that involves the lightsaber. Since Naruto is known for his creative fighting styles, having a lightsaber would just add another dimension to his combat skills.
He would probably be extremely powerful. Naruto is already a very skilled ninja with his own set of abilities like the Rasengan. A lightsaber would add a whole new dimension to his combat skills. He could combine his chakra control with the lightsaber's unique properties. For example, he might be able to infuse his chakra into the lightsaber to make it even more powerful or use his speed and agility to perform some amazing saber techniques that are a blend of Jedi and ninja styles.
His outfit might change a bit. He would need some kind of holster or attachment for the lightsaber. Maybe he would add some elements to his clothes that are related to the Star Wars universe, like a Jedi - like robe or a belt with some Star Wars - style buckles. His hair might also look a bit different when he's using the lightsaber, as if the power of the weapon gives him a more intense look.
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.
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 would start to blend different cultures and values. The Jedi values associated with the lightsaber, like discipline and self - control, could start to influence Naruto. He might become more in tune with his spiritual side, similar to how Jedi connect with the Force. This could lead to new abilities and a deeper understanding of his own ninja powers, as he combines the teachings of the ninja world with those of the Jedi through the use of the lightsaber.
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.
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.
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 creates a lightsaber, it could be related to his connection with Kurama. Kurama's chakra is extremely powerful. It's possible that this chakra could be harnessed in a way that can mimic the energy source of a lightsaber. Naruto, being the creative and adaptable ninja he is, would then figure out how to shape and control this new form of energy into a blade, much like a lightsaber. And he would surely use it to protect his friends and the Hidden Leaf Village.