What if the WSS of an era is recognized when they forge a Black Blade? Think of it as a status symbol.
Ryuma and Mihawk are the only known WSS of their respective eras, and the only unusual thing they share is their Black Blades.
It's said that Ryuma's Black Blade was forged through his history of battle. I wonder if a sword can sense when its wielder has reached the undisputed peak of swordsmanship for their era, and, once satisfied, it turns black.
Zoro could achieve this by directly defeating Mihawk, and his three blades that aided him would be forged black.
In the previous era, the title may have been too contested: Roger, Xebec, Rayleigh, Shiki, Harald, Oden, and perhaps even Whitebeard. After all, WB wielded a Supreme Grade, and we know he and Roger fought to a draw. Some of these legends may never have fought each other decisively (more than a clash) or proven clear superiority.
Unlike Ryuma, who was undefeated, and Mihawk, who is known for his superior sword skills over Shanks.