Powers & Abilities In order to create a Black Blade you have to Forge your Haki into it.

[Dettles]

It's not just someone who has Haki and Skill who can permanently turn a Blade Black, but someone who can actually Forge their Haki into the Black.

 

[TheAncientCenturion]

Probably, Joyboy introduced the idea of storing haki into inanimate objects. I wouldn’t be surprised if some variation of this made black blades.