Tanjiro's sword turning black in "Demon Slayer: Kimetsu no Yaiba" holds a special significance within the story. The black coloration of Tanjiro's sword is due to the unique nature of his sword and its interaction with sunlight.

The black color of Tanjiro's Nichirin Blade is an indication of its rarity and exceptional quality. It signifies that Tanjiro's sword possesses additional properties beyond those of a regular Nichirin Blade. The exact reason for the black coloration is not explicitly explained in the series, but it is implied to be a result of the sword's specific composition or craftsmanship.

When exposed to sunlight, Tanjiro's sword changes color and becomes pitch black. This transformation occurs because the blade's properties react to the presence of sunlight, causing it to take on the dark hue. This unique trait of the sword has been referred to as the "Transparent World" in the series.

The black coloration of Tanjiro's sword is visually distinctive and sets it apart from other Demon Slayer swords. It serves as a symbol of Tanjiro's exceptional potential and the unique path he walks as a Demon Slayer. The black blade reflects Tanjiro's determination, strength, and his connection to the sunlight that drives away demons.

It's important to note that the black color of Tanjiro's sword is a fictional element within the context of the series and contributes to the visual aesthetics and storytelling of "Demon Slayer: Kimetsu no Yaiba."