And in Ryuma's time he was told there was a strong swordsman called King who he wanted to fight. Later we find out Ryuma actually was King but he didn't know, because that's what common folk called him. After defeating the dragon and defending wano for years Ryuma(King) was known as the strongest swordsman on wano(maybe the WSS at time I forgot) and also made shuisui into a black blade.
Jump to present day, arguably the strongest swordsman on wano was King, who Zoro fought. Though we find out that King isn't really a swordsman so unlike Ryuma(who was the strongest) Zoro still has an opponent to defeat, which this time is not a dragon but instead a hawk haha. And many people believe Zoro will make enma into a black blade after defeating king but before leaving wano which would align with ryuma defeating the dragon and making shuisui into a black blade.