That's an incorrect assumption.
Black Blades are the most elite among all weapons. It doesn't matter if someone has a supreme grade, black blades still trump them.
Out of all the characters you mentioned, only 1 has a black blade and even then that guy doesn't have portrayal close to Ryuma.
Zoro is never turning his swords black btw. Oda hasn't written him that way to reach that level.
-> They aren't, that's based on a faulty translation. Supreme Blades hold the most important out of the weapons, hence Oda made sure to bring it up pre-Chapter 100. Ryuuma's black blade is an inferior weapon to the supreme blades.
-> All the characters i mentioned either hold Supreme Blades and/or can rival those that hold them with unknown graded blades or other fighting styles.
Sorry, your hillbilly sword god from an isolated island doesn't hold up with today's top tiers.
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You think owning a supreme grade blade scales above MAKING A BLACK BLADE?
So their feat is that they bought or were given expensive swords… Wow
Being capable of making a black blade has 0 relevance to one's fighting prowess. It simply speaks to so far a degree of haki mastery or rather the length of haki application on to the sword.
Ahhh yea... that's not how it works buddy.
Those that hold supreme blades:
-> The Pirate King
-> The guy who could've been the pirate king
-> Nusjuro, who has been training swordsmsanship for centuries
-> Fujitora, an admiral symbolized by the strongest zodiac symbol, most ambitious marine so far in the series
-> Dracule Mihawk, an individual who is somewhere in the top 3-5 alive right now.
That's Oda's narrative way of establishing blade users at certain tiers. With Supreme Blades being the clear way to do so. Their prowess can only be fully expressed through weapons of the degree of Supreme Blades.