A black blade is the pinnacle of CoA for swordsman
its a common theory, but this has yet to be confirmed. we do not even know if Mihawk made Yoru black or if it was given to him.
while I believe that he did we should still treat this as a theory and nothing more.
I still say its Mihawk. mainly because I can not see much else going to the guy except that.
hes certainly not at the pinnacle of CoO so it pretty much has to be CoA for him.
also, stronger CoA does not necessarily win vs weaker CoA. we have already seen this.
even no-CoA can beat CoA by pure force.