When a demon slayer's sword turns emerald green, ... The blue sword of Giyu Tomioka is the first Nichirin katana shown in Demon Slayer, when he attacks Tanjiro and Nezuko.

In Demon Slayer, a Nichirin sword’s color has more meaning than just aesthetics; it’s linked to the wielder’s breathing, ...

There’s definitely more about the black sword to come in Demon Slayer’s future. ... Giyu Tomioka, controls a blue sword that’s just as powerful and uncontrollable as water itself.