Just like the title asks can all weapons be blacked? Or at least things other than swords?
If ryokugyuโs weapon isnโt a normal sword but a wooden one that on top of thatโs been blackened thatโd be even more dope that it hasnโt broke
As for the how my current guess is it correlates to how much youโre hardening your weapon. Ryokugyu like fujitora with observation haki might constantly using armament haki on his sword to make it that way basically staining the weapon black.