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I think it's because guys like Rayleigh and Shanks simply think of their swords as a tool and nothing more. Zoro treats his swords like they're an extension of himself. Almost like they're sentient beings that require respect.
Rayleigh/Shanks are probably not as dedicated or attached to their blades. Mihawk is probably more in line with Zoro, almost worshipping his blade (Zoro gave his sword a proper burial ffs).
I would assume Oden is of a similar mould so it's interesting that he didn't turn his blade black. Perhaps telling of how difficult a task it is even for the most dedicated swordsman.
I just don't think Shanks or Rayleigh are as dedicated, not that they aren't "strong" enough to achieve it, they're portrayed as haki monsters. But it means a lot that even they haven't done it. This likely means it's too difficult of a task to pull off if your focus isn't completely on achieving it, and for them it might not be worth the trouble of even focusing on, as they can get results in their own manner by focusing in other areas (CoO, CoC, and other implementations of CoA).
Rayleigh/Shanks are probably not as dedicated or attached to their blades. Mihawk is probably more in line with Zoro, almost worshipping his blade (Zoro gave his sword a proper burial ffs).
I would assume Oden is of a similar mould so it's interesting that he didn't turn his blade black. Perhaps telling of how difficult a task it is even for the most dedicated swordsman.
I just don't think Shanks or Rayleigh are as dedicated, not that they aren't "strong" enough to achieve it, they're portrayed as haki monsters. But it means a lot that even they haven't done it. This likely means it's too difficult of a task to pull off if your focus isn't completely on achieving it, and for them it might not be worth the trouble of even focusing on, as they can get results in their own manner by focusing in other areas (CoO, CoC, and other implementations of CoA).
Thet believe black blade is due to haki mastery only
While it is due to going through many battles...
For me there are 2 types of swordmen : those that focus only on swordmanship (they became one with their blades and wants to be the best swordman ) : Mihawk zoro... and those that aren't really focused : Rayleigh Shanks Oden..
And you have the 3rd type which are the non swordmen even tho they can use a blade and some techniques: BM WB kizaru...