I want to preface this thread by saying that this shouldn’t turn into a flame war about why x doesn’t have a Black Blade despite being strong etc..
So, I want to hear all your opinions especially the sword boys here on what advantage that a Black Blade offers. It doesn’t have to be substantiated by manga. It can’t be headcanon I just wanna hear your ideas. These things are more rare than CoC users as far as we know and they’re likely going to be Zoros final powerup EOS so they have to be super special.
What does a Black Blade give Mihawk that another supreme grade weapon won’t give him?
Is it just added Durability?
More DC?
Something Haki related?
It's apparently indestructible.
Ryuma says Shusui wouldn't buckle one bit even against the weight of a Dinosaur
Mihawk implies that Zoro's blade wouldn't get chipped under any circumstances if it were black
Both characters with black blades imply the swords are indestructible so I think that the main thing
Now this rigidity itself basically creates certain combat advantages such as higher AP since the sword doesn't absorb the force of attacks by buckling due to tension and such so all the force of an attack is fully transfered from the swordsman though the sword.
Next is higher block capability since there's no chance of the sword breaking and so yts basically completely up to the swordsman to be strong enough to withstand the attacking force
Basically both these physical stay boosts caused by the indestructiblity basically work the same way swords in real life work. A more rigid sword in real life gives the same advantages just that it's actually impossible for a sword to be indestructible so it's exaggerated how effective this is in One piece... So once again if the sword user is weak then the sword is useless.
Lastly is the Haki stuff. Since we can guess haki has something to do with black blades, I think making a black blade is the culmination of that Breath of all things technique. We've learned that this is a haki technique and it has do with being able to cut everything and also cutting nothing... Now I think when a person truly reaches the highest possible level of this, their sword turns black and from then one they can cut anything they want and nothing they want. Although, only the person who made the black blade can use the sword like this since it's their will that turned the sword black.
This is kinda supported by how Gyukimaru describes Ryuma's fights. Gyukimaru says "Ryuma cut down his enemies in an instant" implying that there wasn't like any 12 hour battles with this guy. Of course this could be because he has like a ridiculously monstrous strength or stupendous speed but we've learned by now that there's counters for all that stuff, observation haki can react to light speed and there's alot of high durability people and items in the world so monstrous strength might not be able to ensure victory in everything... The one thing we know for sure can basically always ensure a win is the breath of all things since its been stated multiple times that you can literally CUT ANYTHING with it... So if Ryuma was a master of the breath of all things and he has this rare legendary swoRd then the two might be related in that you master the breath of all things, your sword turns black.
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This maybe actually 1 of the inconsistencies that Oda makes again...
No people seem to just misunderstand what the statement "Black blades are forged" means.
See Gyukimaru uses the word "Forged" when referring to how Black blades are formed which is a specific word when you think about it since entails very different things from how CoA on Swords is described. What every swordsman does so far is described as "Coating" a blade in CoA whereas Black blades are "Forged".
Now we can agree Haki has something to do with how Black blades are "Forged" so let's use a real life example to see how the word "Forged" is applied. Swords in real life are forged by heat in those giant kilns. As you can see heat is used in the INITIAL stage of he making of the blades to set the components of the sword. Once the forgib is complete the heat dissipates but the sword us fully formed rigid and everything.
In the case of the black blades and Haki, it's similar. Haki is used to Forge black blades in some capacity and when the sword has been fully transformed into a black blades, the Haki dissipates similar to heat dissipating and the sword sets.
Basically what I'm getting at is the the Haki used in the initial formation of the sword and disappears promptly but leaves the sword to set as a black blade similar to how heat is used in the initial formation of swords and disappears promptly but leaves the sword to set into form. Hence Oda's use of the word "Forge" to show the connection between the processes.