After chapter 1033, it is now clear that Zoro has Conq. haki and is part of the elite who can coat it on his body or weapon. At the end of the chapter, we can see the black lightning trail which is a feature of Conq. haki coating but also smoke/steam coming from Zoro’s swords.

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Until now, the only weapon we already saw doing this was Enma. So why Wado and Sandai are now emitting such a steam?

First of all, why is it the case for Enma ? Enma is a sword that sucks ones haki, thus cutting more than needed. So Enma forces the users to infuse 100% or even more of his haki.

In the last chapter, we got to know that from the beginning Zoro was preventing Enma from taking his haki at will because he feared to die by running out of haki completely. Now he realized that it isn’t how it works.

All sword have personalities and have standards regarding which swordsman can wield it. The swordman has to bend the sword to his will, and be worthy of the sword. It is all about being strong enough and having the will that matches the sword.

Mastering Enma isn’t being able to prevent it from drawing out your haki at will but being strong enough to let the sword do as it wants. You have to meet its standards. When Enma isn’t restrained, which is the case when it sucks Zoro's haki against his will and when he let it do it on purpose, we can actually witness this steam coming from Enma.

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Enma sucking Zoro's haki without restriction

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Zoro letting Enma pump his haki on purpose

Since Zoro realized that, he is now going all out, his haki potential is used at 100% by Enma but also by his other swords. The important part is that Zoro is also able to infuse all his haki in his other swords, and thus to draw his full potential even if it isn't Enma.

Enma is a power up to the extent that it is a constant training for Zoro. It is like stamina training, when you train you run out of stamina but you are also building stamina. Zoro is actually using such amount of haki he might die, yet he is also building his haki.

In essence, he can do this training without Enma, but Enma was the trigger for him to do it and also makes it easier for him to go all out, since it is constantly sucking haki out of him.

So this steam seems to apear when the wielder is going all out infusing all of his haki in the blade. I see it as a tap that the wielder can open more or less to coat his haki on the sword. Going all out being the equivalent of comletely destroying the tap.

Now that the context and how all of that works is set up, what is this odd steam coming from all his swords ?

In my opinion, this could be tied to how black blades are made. Black blades are legitimately the most rare thing in One Piece, we only know two of them which are Shusui and Yoru. This lead the community to thing that Conq. haki could be a requirement to forge a black blade, yet why outstanding individuals like Roger or White Beard didn't manage to forge one ?

My guess is that Conq. haki isn't the key to forge a black blade but armament haki alone. However, armament haki has to be used to a massive extent to manage to do it, the requirement is to do exactly what Zoro is doing right now. The blade has to be infused using huge amount of haki during a certain period of time. This could explain why people like Roger or WB didn't manage to do it, since they were maybe used to infuse their weapon mostly with Conq. haki and not much armament.

Also, I strongly believe that the word "forged" is highly relevant. Hitetsu, who is a master swordsmith, said that a sword is "forged" into a blackblade. Firstly, this implies that the process is slow. Enma despite being with Oden for decades isn't one already. But, most importantly the word "forged" makes me think of how a swordsmith craft his sword, more precisely the moment he plunges the hot blade in cold water, thus releasing hot steam or smoke...

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\"Forged\"

Yes, maybe this steam coming from Zoro's swords is litteraly the the sword being "forged". Maybe Hitetsu isn't using a metaphor but describing the actual process and how it looks like.

This is my interpretation of what was going on in this chapter regarding Zoro's swords. Maybe we are witnessing the actual process of "forging a black blade".