Come on dude. Shanks has been the only character that was hyped for his strong coc pre skip. And Roger's coc was hyped by oden, someone who was almost close enough to make a black blade.
Even ignoring that, Kaido, the supposed strongest creature in the world, hyped up Roger and Shanks and had them in his top 5. You think he does that to dudes with weak coc?
Where as, Mihawk has NEVER been hyped up about his coc AT ALL. The only reason we even know Mihawk has it is cause Zoro has it.
It's pretty clear that acoc isn't what gives someone a black blade. It's gonna be some philosophical shit like Miyamoto Musashi and his becoming the void. Basically, they have to become the sword itself and be one with the sword to get a black blade. And Shanks and Roger and WB and whoever else just saw their swords as just tools to channel their power.