I've been reading a lot of threads about Zoro not having ACoC, and I think that they might be on to something.
My theory is that Zoro doesn't have haki at all, but rather is a user of the yami yami no mi.
Look at that black cloud that covers Zoro's sword. Anything feel familiar about it?
There is only one power in One Piece that allows the user to create black clouds and that is the yami yami no mi.
But how could there be two users of the yami yami no mi? Well as we know, the yami yami no mi has the ability to suck in people. I think this is likely how Zoro got his yami yami no mi powers. Over the timeskip, Zoro got sucked by Teach and after spending a lot of time inside Teach's body, eventually gained some of his yami powers.
There is precedent for such a thing in one piece. While there are multiple vegapunks, all who can connect to Punk Records, only one of them holds the brain brain fruit. You also have Nami, who uses Zeus despite not being the user of the soul soul fruit. Zoro and his Yami are similar.
Anyways, I think Zoro getting sucked by Teach is a reference to the story in the Tanakh(hebrew bible) about Jonah.
https://en.wikipedia.org/wiki/Jonah
As you might know, Jonah was commanded by G-d to go to Nineveh, but Jonah didn't want to do that and instead sailed to a different city. A storm troubled the ship, the sailors threw him overboard, and Jonah was swallowed by a big fish, which vomitted him out on dry land. After which, Jonah accepted his destiny and traveled to Nineveh.
I think at some time during his training, Zoro lost his will to become WSSM. This might have happened after he lost his eye. He decided to leave Mihawk's island and return home. But Teach saw him, sucked him with the yami yami fruit, and kept Zoro inside his body for 3 days and 3 nights. After which, he threw him back up on Mihawk's island. Zoro probably gave Teach diarrhea or something so he decided to let him go.
Ever since, Zoro has been able to coat his sword with darkness, a skill he uses to fulfill his destiny. One he cannot escape from.
My theory is that Zoro doesn't have haki at all, but rather is a user of the yami yami no mi.

Look at that black cloud that covers Zoro's sword. Anything feel familiar about it?

There is only one power in One Piece that allows the user to create black clouds and that is the yami yami no mi.
But how could there be two users of the yami yami no mi? Well as we know, the yami yami no mi has the ability to suck in people. I think this is likely how Zoro got his yami yami no mi powers. Over the timeskip, Zoro got sucked by Teach and after spending a lot of time inside Teach's body, eventually gained some of his yami powers.
There is precedent for such a thing in one piece. While there are multiple vegapunks, all who can connect to Punk Records, only one of them holds the brain brain fruit. You also have Nami, who uses Zeus despite not being the user of the soul soul fruit. Zoro and his Yami are similar.
Anyways, I think Zoro getting sucked by Teach is a reference to the story in the Tanakh(hebrew bible) about Jonah.
https://en.wikipedia.org/wiki/Jonah
As you might know, Jonah was commanded by G-d to go to Nineveh, but Jonah didn't want to do that and instead sailed to a different city. A storm troubled the ship, the sailors threw him overboard, and Jonah was swallowed by a big fish, which vomitted him out on dry land. After which, Jonah accepted his destiny and traveled to Nineveh.
I think at some time during his training, Zoro lost his will to become WSSM. This might have happened after he lost his eye. He decided to leave Mihawk's island and return home. But Teach saw him, sucked him with the yami yami fruit, and kept Zoro inside his body for 3 days and 3 nights. After which, he threw him back up on Mihawk's island. Zoro probably gave Teach diarrhea or something so he decided to let him go.
Ever since, Zoro has been able to coat his sword with darkness, a skill he uses to fulfill his destiny. One he cannot escape from.