Sounds like an even greater feat, if on top of extreme power you need destiny/being the chosen one. The problem however is, that Mihawk doesn't have any special "destiny" portrayal. Mihawk is largely irrelevant to the story, he is just a power house chilling in his castle and waiting for Zoro to surpass him.
Well, the full moon might be one element, but it is still Zoro who needs to have the power. I mean, I could use your argument of there only being 2 swordsmen in centuries with a black blade, while a full moon appears once a month. Maybe you could say that it turns black when you behead a dragon at full moon, but in my opinion that is way too specific. Or is it when swordsmen that are related to the moon, can turn their blade black at full moon when beheading a dragon? Still sounds too specific.
It was said that Ryuma turned his blade black in the course of his many battles. Not that it turned black the moment he killed the Dragon. And Mihawk killing a Dragon at full moon also sounds too specific, plus, how is he related to the moon? As I said, he has no relevance to the story other than waiting to be replaced by Zoro as the WSS.
Yes ofc, it's gonna be Zoro's Feat, but i meant that the Full Moon must bare witness. And yes it's extremely specific & that's why it's super rare.
And you're really underestimating Mihawk's Involvement in Story
The reason Oda never speaks about him is for same reason All Blue is barely mentioned or Cure that Chopper is looking for or we barely seen anything from Elbaf Kingdom or learned anything about Void Century ... etc
Mihawk is one of MC's End Dream just like those i mentioned above & Oda will only settle for very minimal hints for now, cuz these stories are almost completely reserved for Final Arc. Trust me, Mihawk's Backstory is a huge deal & extremely important in story
Just to give you an idea of what i'm hinting at
Shirahoshi is Special right? Why? Because she was born with Poseidon Powers
Who gave her those Powers? We can say she was Fated to be Poseidon
Can't we say the same thing about Big Mom? Why was she born like that? Doesn't it sound like she was chosen by Fate?
What about Vegapunk? Why is his Intellect described to be 500 Years ahead of Humanity?
I can give you other examples, but i want you to understand that there are certain Powers in One Piece or Unique Treats that are Fated for certain people
Luffy isn't the only one who was fated to eat Nika's DF
There are many Characters who are chosen by Fate to carry certain Unique Abilities
And a Black Blade is one of them, it's a Sword forged under Full Moon granted to specific Swordsmen who will be part of The Dawn
Think about it:
Luffy have Sun God Nika DF
Zoro is gonna have a Black Blade
Sanji have Flaming Body even though he isn't Lunarian
Franky had Pluton Blueprints
Usopp's Lies turn into Reality
Robin is only person who can read Poneglyphs
Jinbe is only Non-Merfolk who can talk to Fish (As Crocodile pointed out)
No need to say anything about Brook
... etc
Don't you see a pattern here? These People aren't Normal, they aren't just random dreamers, they are clearly Chosen by Fate