In the "Demon Slayer: Kimetsu no Yaiba" series, Tanjiro Kamado's sword turns black after his transformation into a demon. This change in the appearance of his sword is due to a unique property known as "Black Nichirin Blade."

A Black Nichirin Blade is a rare and special variation of a Nichirin Blade, typically associated with demon slayers who have turned into demons themselves but retained their human consciousness. When a demon slayer becomes a demon but maintains their humanity, their Nichirin Blade turns black instead of the usual color associated with their abilities.

The black color of the blade symbolizes the duality within Tanjiro—his struggle between his human side and his demon nature. Despite being a demon, Tanjiro's strong will and determination allow him to control his instincts and remain true to his human values.

The black sword serves as a visual representation of Tanjiro's unique situation and the internal conflict he faces as he fights against demons while being one himself.