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 ...

Crafting a full-scale, 20-foot-long replica of Guts’ legendary Dragon Slayer, this cosplayer has brought the most absurdly massive anime weapon to life in a way that even the Black Swordsman ...

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.