well hes the #1 swordsman so its important to highlight his sword outright
Yes because in character introductions Oda highlights their most powerful abilities/reasons for their hype.
For Whitebeard, it was the Gura Gura & title.
For Kaidou, it was his toughness & title.
For Mihawk, it's his Black Blade & title.
For Shanks, it's his CoC.
For all the Admirals? It's their powerful Devil Fruits.
Just like how for Ryukogyu, it's his extremely powerful Logia which was highlighted again & again.
well yeah because its not greenbulls time yet, you cant expect all of his arsenal to be revealed outright
that sword is there for a reason and it was black for a reason and it didn't have a sheath for a reason
And that reason is to bait everyone to talk about it lol. That's how Oda's been peddling this manga for a while.
There's still zero panels/statements/confirmation that it's a Kokuto btw.
And it isn't even some random island that Ryukogyu pulled up onto... the man came to the motherfucking Land of Swordsman, where they literally worshipped Ryuma in the form of his Black Blade yet not a single word was mentioned as he walked through the land wearing a similar blade? Hell nah.