It's CoA related. Did Mihawk know Zoro had CoC back then? Definitely not.
Mihawk connected black blades with coating your swords with CoA. If it was CoC related then Shanks and Roger would have one too.
If it is CoA training related then I don't expect Roger and Shanks to have it, but a stoic dedicated swordsman like Mihawk would.
Naturally Ryokugyu had to train his CoA to godly levels to become a top tier with a couple of twigs.