Zoro’s eye has been closed with a large scar across it since we’ve seen him after the timeskip. The only other notable swordsman to have the same scar is Silvers Rayleigh, however his eye is opened.
In traditional pirate history, pirates wore eye patches in order to see inside of a dark ship. When they’d enter the ship, they would switch their eye patch so that the eye that is better adjusted to the dark.
One Piece is a series about Pirates after all, so it wouldn’t be unlikely that Oda used this well known piece of history as a basis for Zoro’s closed eye, as well as the DARK King Rayleigh.
Now for how I think it went down.
Rocks D. Xebec likely was the user of the Yami Yami no Mi. When the Roger Pirates faced off against Xebec’s crew, it’s more than likely that it wasn’t their first clash. In order to counter the abilities of the fruit, Rayleigh had to keep his eye shut until their fight. When opened, Rayleigh could not only see through the darkness, but cut through it too. With the help of Roger and Garp, Rocks was defeated, Rayleigh was titled the Dark King, and he never had to keep his eye shut again.
Now comes Zoro. Zoro got his scar when training with Mihawk. This part of the theory can go 1 of 2 ways, both of which are equally plausible.
The first idea is that, after Rocks was defeated, Rayleigh trained another swordsman to counter the powers of the Yami Yami no Mi. That swordsman being Hawkeyes Mihawk. After witnessing what took place at Marineford, he went back to train. Only to find a more worthy opponent for Blackbeard. Mihawk scars Zoro and then trains him to be even better without it. So when the time comes that he must face Blackbeard, he will be ready.
Alternatively, Mihawk could have just figured this out without the help of Rayleigh and still went on to train Zoro and close his eye.
Most people have the idea that Luffy will 1v1 with Blackbeard and it’s just very unlikely. An awakened ability of Luffy’s would be able to counter the Gura Gura no Mi, but not the Yami Yami no Mi. That’s where his first mate comes in. Oda has shown us how important his crew is, and for Luffy to defeat the “main bad guy” without the help of his first crewmate would be hollow and “un-Oda-like”.
Just a little kicker for the above, if it turns out that Blackbeard is in possession of a mythical zone Kraken fruit, it could turn this fight into a monster trio bash. Sanji couldn’t help himself not to fry up some good seafood.
Also, if the theory about Luffy having 12 crewmates is correct, that leaves 10 Titanic Captains to face 10 straw hats, and for Blackbeard to face the Monster Trio.
That’s the end of my theory. Let me know what you think!
Theory by TallMidgetCuckJesus (https://www.reddit.com/user/Milemanske)