In a series about ancient demons lurking in the shadows and powerful swordsmen trained to stop them, it's surprising that one of Demon Slayer: Kimetsu no Yaiba's biggest mysteries is the color of Tanjiro Kamado's sword. The Nichirin blade is a Demon Slayer's best friend, as it is one of the only ways that a demon can be killed. The swords also carry a personal touch because the metal of a Nichirin blade will change into one of several colors depending on the wielder. When Tanjiro's sword unexpectedly turned black, it may have been the first sign that he was in for a remarkable journey.
Nichirin blades are capable of killing demons via decapitation because the mountain where the metal is mined is constantly drenched in sunlight, the fatal weakness of all demons. Decapitation by any other means will not kill a demon, making the swords vital to the Demon Slayer Corps. Once Tanjiro survived the Final Selection, the Corps' Swordsmith, Hotaru Haganezuka, delivers Tanjiro his personal Nichirin blade to officially make the young Kamado a member of the Demon Slayer Corps.
When Haganezuka sees Tanjiro's reddish hair, he becomes hopeful that he will get to see a bright red Nichirin sword. However, once Tanjiro unsheathes his weapon, the blade turns a dark black, to the concern of Urokodaki and the anger of Haganezuka. Noting their reactions, Tanjiro worries that the color is an omen of bad luck, which Urokodaki answers isn't necessarily the case. That Tanjiro travels with his sister who is a demon who doesn't eat humans is already strange among demon slayers. But, he becomes even more anomalous as Urokodaki mentions that black swords are rare.
The unusual color is still an indication of Tanjiro's talent though because Nichirin blades do not change color for a wielder who is unskilled with a sword. Tanjiro pushed himself beyond his body's limits for two years while training to become a Demon Slayer, as he is not naturally gifted. The black color may be a testament to Tanjiro's incredible will, or it could be a sign that he is destined for greatness. Despite the significant personal meaning behind the color of Nichirin blades, they all are capable of killing demons so at least Tanjiro is not disadvantaged due to his sword's unusual shade.
When Tanjiro boards the Mugen Train to join Kyojuro Rengoku for a mission, the Flame Hashira offers to train him himself, asking Tanjiro the color of his sword. Perhaps ominously, Kyojuro bursts out laughing upon hearing that it's black, describing the color as unfortunate. Kyojuro has never heard of a Demon Slayer with a black sword becoming a Hashira, citing that they tend to struggle to decide on a Breathing Style to master.
Kyojuro's explanation regarding the color of Tanjiro's sword does make a certain amount of sense, considering that Tanjiro, too, has found himself caught between multiple Breathing Styles. Initially, a user of Water-Breathing, Tanjiro has come to rely on the Hinokami Kagura, later revealed to be a Sun-Breathing technique. Tanjiro currently holds the distinction of being the only Demon Slayer in the series with a black sword and the only one who uses multiple Breathing Styles. Still, the Flame Hashira's words aren’t overly helpful regarding why Tanjiro's sword is so unusual.
It's possible that black swords are so rare because the users often die young, weakened by their lack of mastery of a single Breathing Style. By all rights, Tanjiro should be dead several times over by now, considering his knack for being in constant danger even more so than the average Demon Slayer. He is constantly attacked by members of the Twelve Kizuki, the most powerful demons in the world, and it's a miracle that Tanjiro has survived all that he has. He's even the first Demon Slayer to have an encounter with Muzan Kibutsuji, the Original Demon, and live.
Tanjiro has always had a touch of destiny about him, so his unusual sword and knowledge of Sun-Breathing, the original and most powerful Breathing Style, may be signs that he is destined for greatness. It may just be that Tanjiro's black sword is nothing more than a way to show that he is unique, but only time will tell if any further meaning will be discovered about his sword. After all, Ubuyashiki, leader of the Demon Slayer Corps, says that Muzan's encounter with Tanjiro is the first time he's ever shown himself to a Demon Slayer, meaning that the tide is finally turning and Tanjiro just might be the reason why.