Perhaps it's a result of a new chakra - based experiment. Some ninja scientists might have been working on a way to combine different elemental chakras, and Naruto accidentally becomes the test subject and gains the ability to control fire. Or it could be a gift from a powerful spirit or deity that he encounters during one of his adventures.
It could be from some sort of ancient ninja training that was lost and he somehow discovers it. Maybe there's a hidden scroll or a hermit who teaches him this fire - controlling ability.
One possibility is that he inherits it through some sort of bloodline connection that was previously unknown. Maybe there's a distant relative who had the ability to control sand, and it somehow got passed down to Naruto.
One possibility could be some sort of new training he undergoes. Maybe he discovers a hidden jutsu scroll that teaches him how to control and produce fire from within his body. This could be a long - lost technique from an ancient ninja clan.
It could be a result of some new form of chakra manipulation. Maybe he discovers a way to combine wind and earth chakra in a unique way to create and control smoke.
Well, in some fanfictions, Naruto's water - controlling ability could be a gift from a powerful sage. This sage, seeing Naruto's potential and his need for new powers to face upcoming threats, bestows upon him the ability to control water. Another origin could be that Naruto accidentally absorbs the chakra of a water - using enemy during a battle, and this chakra somehow melds with his own, giving him the power to control water.
One possible origin could be some kind of ancient bloodline. Maybe Naruto has a hidden bloodline that was passed down through generations and finally awakened, allowing him to summon angels. Another option is that he got this ability through a special training or a quest. He might have found a hidden place or an old sage who taught him the art of summoning angels.
He could use it for defense. For instance, creating a huge storm to block enemy attacks or a thick fog to hide his village from invaders.
In some fanfictions, it could be due to a genetic mutation or an unexpected side - effect of a jutsu he uses. For example, if he experiments with a new form of chakra manipulation, it might accidentally give him the power to summon swords. Or maybe it's a gift from the Sage of Six Paths in a different interpretation of the Naruto universe.
It could be a result of some kind of rare genetic mutation. Maybe there was a dormant gene in his Uzumaki bloodline that got activated under certain circumstances, like during a near - death experience or a powerful chakra surge.
It could also be a result of a powerful curse or blessing. For instance, he might be cursed by an old enemy with the ability to control time as a side - effect, but he then learns to master it. Or, it could be a blessing from a sage or some other powerful being who sees Naruto's potential and gives him this unique ability to help him save the world.
One possibility in this fanfiction is that Naruto is exposed to a powerful and otherworldly energy source. This energy could be from a different dimension or a long - forgotten relic. When he comes into contact with it, his body absorbs and adapts to it in a way that allows him to control black fire. This new power also comes with its own set of challenges and responsibilities for Naruto as he tries to integrate it into his existing set of skills.