In the popular shonen television show Demon Slayer, demons are supernatural entities that are practically eternal. Demons can rejuvenate, hence they cannot pass away naturally. The nichirin sword, a unique weapon, is the only weapon other than sunlight capable of defeating demons. These unusual swords, made from scarlet ore and red iron sand, are used by demon slayers. The materials are the only ones that can successfully decapitate a demon because they absorb sunlight.
Swords made by the Nichirin school are known as color-changing swords because the blade will change color to reflect the user. However, only two known users of Demon Slayer have had a black nichirin blade, which is said to be unlucky.
What in Demon Slayer Do the Nichirin Blade Colors Stand For?
The color of a demon slayer's nichirin katana is often associated with the user's breathing technique. However, the user must have a certain level of skill or else the blade won't change. This is seen with Kyojuro Rengoku's younger brother, Senjuro, who gave up on becoming a demon slayer due to his inability to change the sword's color.
Flower Breathing users Kanae Tsuyuri and Kanao Kocho have light pink blades. Flame Breathing users' blades tend to be a bright red, and Wind Breathing turns the blade green. Zenitsu Agatsuma's sword is unique -- his blade changed to have a yellow zig-zag pattern, reflecting his lightning technique.
When Demon Slayer protagonist Tanjiro Kamado received his sword from Haganezuka, the swordsmith eagerly hoped the blade would turn a bright red. He had stated that Tanjiro's bright eyes and red hair were signs of good fortune. However, when the sword turned black instead, Haganezuka became enraged because he wanted to see a red blade. The Demon Slayer Corps doesn't have much information about swordsmen with black blades; in fact, they are said to die young due to a lack of skill.
Why Did Tanjiro's Sword Turn Black?
Along with an implied a lack of skill, it is also said that swordsmen with black blades struggle to master a breathing style. Tanjiro utilizes Water Breathing, but unlike Tomioka, his sword isn't blue to reflect the breathing style. This is because he also can use the Hinokami Kagura technique. The dance Tanjiro learned from his father is the first original breathing style -- Sun Breathing -- and has been passed through the Kamado family for generations.
In the Demon Slayer manga, the only other known individual to master Sun Breathing was Yoriichi Tsugikuni -- who had a pure, deep black katana like Tanjiro. He was known as a legendary demon slayer and is said to have invented the first breathing techniques. All known breathing styles that came afterward are derived initially from Sun Breathing. The color black can absorb the most sunlight, making the Sun Breathing technique the single most deadly weapon against demons.