In One Piece we've learned to rate swords by their quality as Meito graded swords. Mihawk and Whitebeard both had swords in the top Meito class, the best 10 swords in the world, and we knew this was because their swords were the strongest in the world.
So why is Zoro suddenly so much stronger with Enma? It's the same Meito grade as Shusui. It should be a sidegrade, or somewhat close in power, and not this overwhelming upgrade like it's being presented. I no longer understand what the different sword grades mean anymore.
Can someone explain why two swords of the same meito class could be so different in power? Is the meito class a flawed indicator of strength, and it was just coincidence that WB and Mihawk both have swords in the top 10 Meito class?
Enma and Shusui are both O Wazamono or Great grade swords. Enma has shown to be far stronger than Shusui. As stated on the wiki page, Enma is the only weapon in the series the directly effects the user Haki on its on accord, which means that Enma is the only weapon so far with a unique ability. Shusui is already a "Black Blade" and we've seen that in the hands of Zoro that it can further be blackened with Haki/Busoshoku Haki. My guess and this is my theory, before becoming a permanent "Black Blade", Shusui was one of the Ryo Wazamono or Skilled grade swords. The lastest chapter gave us new information. When Enma is turned into a permanent "Black Blade", it will rise in rank to Saijo O Wazamono or Supreme grade sowrds. Even though there the same class, Enma has yet to become a "Black Blade" when Shusui already is, and I highly doubt we've seen Enma used on a mastered level yet so im sure even greater feats than last chapter are waiting. Besides Zoro's feats from last chapter are greater than anything he's done up til now in the enitre series in my opinion and that was just an accident with just one sword. Im sure Enma will become Zoro's favorite sword once he's mastered it and he will have a Saijo O Wazamono or Supreme grade sword to battle Mihawk with.(Cman12 (talk) 05:49, September 18, 2019 (UTC))
So you're saying that the Meito grade is dynamic and can increase when it becomes a black blade? That would make sense. It would mean that Enma is technically a grade better than Shusui, the Shusui only being O Wazamono because it was blackened. This would explain why Enma is stronger, despite also being O Wazamono. Enma reached O Wazamono grade before being blackened and is actually a Saijo O Wazamono grade when blackened.
However, this theory implies one of two scenarios.
Either a) by the same logic Shusui is technically a grade weaker than Enma, Zoro would still have a weaker blade in Enma than Mihawk's Yoru and WB's Murakumogiri, both blackened Saijo O Wazamono swords, which should be stronger than blackening an O Wazamono sword like Enma. Then this theory would not make sense, since we know Zoro is going to become the best swordsman in the world, and he should have the best sword to do it. He can't not have at least one sword which is considered on par with or exceeding the best in the world.
Or b) if Zoro finally has a sword in Enma that's on par with Mihawk and WB, then it implies that both Mihawk and WB also began with O Wazamono blades that became Saijo upon blackening, and no Saijo O Wazamono grade swords existed that aren't black blades. Then the theory could be true. The only problem is we know this is not the case. The Nidai kitetsu is not black and is already O Wazamono, which would imply the Shodai kitetsu is Saijo O Wazamono and unblackened.
I guess the question becomes: Does a non-blackened Saijo O Wazamono blade exist? If the answer is no (and the Shodai is already blackened somehow), then the only path to Saijo O Wazamono is to blacken an O Wazamono blade, and this theory is probably true. If the answer is yes, then this theory may not be true, because Enma could still be too weak to be the strongest sword in the world. Then it would remain unresolved why the Enma is so much stronger than Shusui.
The only third possibility is that Enma is not Zoro's final sword, and this theory is true; in other words, that Zoro has to yet to find a non-blackened Saijo O Wazamono sword, which upon blackening would reach a new grade of sword that doesn't exist yet. That would be pretty hype for the strongest swordsman in the world. Probably the Shodai Kitetsu would be a great candidate, considering Zoro is definitely going to replace a crappy fourth tier sword like the Sandai Kitetsu, and it would logically follow to be replaced by its brother the Shodai, who seems to have already been implied is an unblackened Saijo O Wazamano Mieto. It just just lacks the proper mythos to fill his #1 sword spot, as compared to Enma, which scratched Kaido and was wielded by the shogun of all Wano, or the Wado Ichimonji, which is his past's heirloom. There may not be enough time to do all of this, considering Zoro took over 10 years for his last 3 swords.
Okay, there was a translation error in the last chapter (955), and apparently it specifically states that blackening the blade will cause it to "rise in rank." This would support your theory. Then I guess that would mean Both Yoru and Murakumogiri were O Wazaomono grade and upgraded to Saijo. That means that either Enma is going to be Zoro's ultimate blade to match Yoru, or he will replace the Sandai Kitetsu with the Shodai Kitetsu and blacken it to reach a new grade that's never been heard of before. Unless the Shodai is the same grade as the Nidai but was already blackened.
I like the idea of Shodai upgrading more though, not only because it would be more hype for the strongest swordsman in the world, but also because it would be Zoro's personal sword raised solely by his own strength. Just like Shusui was Ryuma's, Enma was Oden's while the Wado Ichimonji was his dojo's. It would be nice for Zoro to raise one of his swords completely on his own.