The swords of Demon Slayer: Kimetsu no Yaiba are all highly personalized to their wielders. Inosuke dual-wields serrated edges to hack through demons and trees alike. Zenitsu’s blade is marked with the lightning of his characteristic Thunder Breathing technique, Thunderclap and Flash. But at first glance, Tanjiro’s blade doesn’t really match his Water Breathing form or his jubilant personality.

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Not just any sword can slay a demon, so the Demon Slayer Corps utilizes special materials with unique properties that give each sword a flashy appearance. A Demon Slayer blade is called a Nichirin Sword, and a unique one is forged for every member. Forged from Scarlet Ore from a sunbathed mountaintop, the blade contains sunlight that helps the edge slice through a demon’s neck with ease (well, most of the time).

Tanjiro’s black sword

Each ore is picked by Demon Slayer Corps initiates after passing final selection, though the final appearance of the blade is determined by the swordsmith and wielder. Tanjiro’s iconic black blade was forged by the fiery Hotaru Haganezuka, a dango-loving swordsmith who works with the Demon Slayer Corps.

Hotaru excitedly presented Tanjiro’s Nichirin Sword to the recruit just 15 days after passing his trials. The katana’s unique handguard is a black ring with wheel-like spokes, perhaps alluding to the cycle of reincarnation that the characters believe in. Because of Tanjiro’s red hair and eyes, Hotaru hoped the “Child of Brightness” might draw a red sword, but was surprised when the unsheathed Nichirin blade turned black in Tanjiro’s hands. Another swordsmith remarked that he’d never seen that shade of black, however.

What does this mean?

Nichirn swords require sufficient prowess to change color, but only skilled Demon Slayers make it into even the lowest ranks of the Corps. The weapons are one of humanity’s only tools to fight demons, forged by masters of their craft. Still, little is known by the Demon Slayers about black swords. Their wielders, we’ve heard, don’t survive long enough to find out.

Still, Tanjiro has a somewhat unique fighting style, and a rare blade seems appropriate for the character. Black swords are a recurring motif in anime as well, often associated with lone wolves like orphans or those carrying a burden. Many great swordsmen, whether protagonist or antagonist, have wielded black swords across the history of shonen battle anime. 

Throughout the Entertainment District Arc and Swordsmith Village Arc, we discover more about Tanjiro’s background and the reasoning behind his black sword. Black blade wielders are said to die young due to a lack of skill, but it’s also prevalent that they struggle to master a Breathing Technique. For Tanjiro, he can use Water Breathing, but his blade never turns blue to reflect this. This is because he can also use the Hinokami Kagura technique, which is consistent with Sun Breathing, his father’s Breathing Style.

In the manga, the only other individual known to master Sun Breathing was Yoriichi Tsugikuni, who also had a pure black blade like Tanjiro. Tsugikuni was said to have invented the first-ever Breathing Styles, including Sun Breathing. It’s also worth noting that the color black insulates the most sunlight, making it the most deadly weapon against demons.