Zoro used a black blade all alone and after he changed it with enma he became stronger, its not relative to strength, changing a blade to black is still an unknown process
Zoro didn't forge Shuisui into black blade.
There is no evidence that Zoro became stronger with Enma.
There was never a statement from Zoro nor anyone saying Zoro became stronger with Enma.
Enma was even nerfing Zoro's haki control.
Unleashing huge amounts of haki is something that Zoro already know. He has Ashura since Enies Lobby.
Mihawk already stated any blade can be forged into a black blade while teaching Zoro haki.
Both Ryuma and Mihawk were said to have countless battle.
As we know, haki bloom in battles.
Forging a black blade is definitely relative to strength.
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shanks and roger hit without cutting + nasjuro use his zoan form to bite people and king hit with fire and pure strength and he said himself he was not a sworsdman and wb hit with his fist, no one of them are sword based people even without sword they still can beat ur ass, what about mihawk ?
Zoro literally swung his sword at a branch and did not cut, pre-timeskip back in Alabasta. And it is one of the earliest sword technique that's been introduced.
King is not a swordsman.
WB is not a swordsman.
Roger, Shanks, and Nasujuro are swordsmen. They primarily use sword as their main offensive weapon.
Roger and Shanks literally have used only swords on all of the battles they've been shown.
Even Shanks' wifi haki has to use his sword lol. Couldn't he have just pointed with his finger?
edit:
**Roger was shown to have used a pistol. But you get my point.