You really believe that? The designs and everything, it doesn't look ordinary sword from any angle.
Anyway, If that was true, than Shusui another Black Sword would have been the current Best sword in Onepiece. But Yoru still has superiority to another Black sword, concluding it was always a Grade sword and this case of random sword rather can be made for Shusui, if things work that way for Swords.
Whatever the specifics are, the story is telling us that over and over again that Swordsmen make swords they use useful.
Mihawk says any sword can become a black blade. If the theory that black blades are literally indestructible is true, then that means that any sword can become indestructible as long the swordsman is good enough.
Gyukimaru attests Shusui's fame to it having turned black from Ryuma’s "countless battles" with it and not because it was made at high quality
Tenguyama says swords literally rise in rank according to how good the swordsman is so once again if the swordsman exceeds the sword, the sword can be forced to rise to to the swordsman's level.
Even how Zoro deals with swords shows that higher grade doesn't help anything if you aren't good enough. Zoro fails to do 108 canon, a technique he had mastered in Skypiea, when Shusui creates a power imbalance. When Zoro does master Shusui post time skip, there's NO DIFFERENCE between Shusui and any other sword he has. We even see him do 1080 canon and even Sandai Kitetsu which is two ranks below Shusui does as much damage as Shusui.
See how that works? Shusui forced Zoro to train to get stronger to be able to use it properly, but the strength Zoro got after this training doesn't disappear when he's not using Shusui. Everything he can do with Shusui after reaching a certain power level he can then do with Sandai Kitetsu and Wado. Shusui alone remains a better sword than Sandai Kitetsu but now that Zoro is strong enough to make Shusui behave how he wants, everything he can do with Shusui he can do with Sandai too.
This is why people didn't understand Enma. Enma is once again forcing Zoro to train to have a certain power level to use it but after training, Everything Zoro has learned in order to use Enma properly is still fully applicable to Every other sword Zoro has. This is why Oden can cut Kaido with both Enma and Habakiri, because he's strong enough for the difference in the two swords to no longer matter.
The test to prove Zoro has mastered Enma is to just cut a tree with enough force and haki to only cut that tree and nothing else. Zoro can ALREADY DO THIS with any other sword he has. So essentially Zoro is training to make Enma behave like any other sword he has. Therefore in the end, Enma will in fact behave and do everything just like Sandai Kitetsu which is two ranks below it because Zoro would be strong enough for their to not be a difference. A good swordsman makes any sword he wants do whatever he wants it to do.
