I think there was no WSS before Mihawk.
Mihawk probably didn't get to challenge the strong pirates in Roger's era like Roger himself or Rayleigh, since Mihawk was still too young back then like Shanks.
When Roger got excuted, Mihawk was probably just an upcoming pirate at the time.
Given also that no other black blades were mentioned aside Shusui and Yoru, I think the WSS title was made and given to Mihawk once the world saw he forged Yoru into a black blade. Now whether he defeated someone, something, or entirely different when he turned Yoru into a black blade is what we have yet to know.