Tengu told Zoro, that Enma has still to reach a higher rank by becoming black, implying that Oden failed to achieve that level. It puts Oden beneath those who forged a black blade by default.
- no side powers like DFs, guns, martial arts, advanced CoC (if it exists)
- a swordstyle named after him (Oden Nitoryu) which he even teaches his scabbards
It's the exact same fighting style like Mihawk and Zoro have; the black blade is the only thing that would make a difference between them.
Now one might ask "what about Roger and Whitebeard, not even they have a black blade". Good point, but I believe it doesn't make sense to use Roger as an example, since we are not aware of his full power. Is there an adanced form of CoC he uses as a compensation for the Gura? Who knows, we might find it out in Luffy vs. Teach, when Luffy's DF will be blocked by the Yami fruit.
You could also make a case, that a black blade is forged not only by having extremely strong CoA, but also by mastering "the way of the sword", which is the ability to "cut everything and nothing as you desire", by letting the Haki flow through your blade in a specific way to become one with it. An ability Roger was not proficient in, but rather made his CoA overflow in his blade to throw barriers (just like Roger attacked Oden with a blunt attack without cutting him).
That's why Zoro asked Mihawk "teach me the way of the sword". What was Mihawk's answer? "every blade can become a black blade like mine". Though Mihawk himself also said, that mastering Haki is a crucial part of it, meaning that your CoA has to be of a very high level too. In fact, it has to be advanced Haki, because that's how Hyogoro described the ability to become one with your blade; by letting the Haki flow out of your blade and cut everything.
Oden, Zoro and Mihawk, their fighting style is exactly the same. There are no side powers. No Haki barriers form of fighting, no (assumed) advanced CoC, no guns, no DFs. Just strict swordstyle, just cutting with every technique. What did Oden do to Kaido when almost killing him? Exactly, cutting technique. It's what Oden's, Zoro's and Mihawk's ability is based on.
So this is why the black blade is what makes the difference. If the black blade doesn't even matter among those who have the exact same way of fighting, then it's nonsensical to introduce black blades in the first place. What would change if Zoro turns Enma and his other blades black?
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