Shusui -- the sword became black

[ahill]

The guy battling Zoro currently (forgot his name) stated that the sword Shusui has grown into a dark, black blade... what did he mean? Maybe that the sword, the more is embued with Haki, acquires the dark colouration characteristic of the Armament Haki?

 

[SSJ2]

I think he was spouting bs.

 

[Captain Cadaver]

It being due to Busoshoku Haki would make sense with how the blackness stopped being an exclusive trait of Luffy in Punk Hazard when Haki user magically started having this trait (even though it being an exclusive trait to Luffy due to how vulcanised rubber works irl would make more sense). Given Wano being inspired by Japanese folklore and period piece tropes, it probably has to do with the blood shed from Shusui though.