In the "Demon Slayer: Kimetsu no Yaiba" series, a black Nichirin Blade holds special significance. It represents a unique circumstance where a demon slayer has transformed into a demon but managed to retain their human consciousness and willpower.

The black color of the Nichirin Blade symbolizes the duality and inner struggle within the wielder. It signifies the coexistence of their human nature and their newfound demon abilities. Demon slayers with black Nichirin Blades are exceptionally rare and often possess remarkable strength and resolve, as they resist the urges and instincts typically associated with being a demon.

Having a black Nichirin Blade sets these demon slayers apart from regular demons, as they continue to fight against their own demonic nature while upholding their purpose as demon slayers. Their black blades become a visual representation of their unique circumstances and serve as a reminder of their ongoing internal conflict and determination to protect humanity.