The idea being spoken of here is the permanent blackening of a blade. Constant imbuement with Armament Haki, and potentially even Conqueror's Haki, might allow one to turn their sword black.

In Anime Last Stand, Black Swordsman is a powerful Celestial-rarity unit focusing on landing heavy strikes while inflicting Bleed and Hemorrhage status ailments. The character is based on Guts ...

Vagabond is a beautifully illustrated and fictionalized biography about Miyamoto Musashi, the swordsman and writer of The ...