Like you said even people with monstrous haki didn't manage to make their blades black (Roger, WB etc.) so first of all it can be discussed if it is a CoC feat or a CoA feat. Most likely it is a coa feat or at least mainly because we know Zoro specializes on CoA (Luffy CoC and Sanji supposedly CoO), even Mihawk talked about making the sword black, see a dent in the sword as a dent in the swordsman pride etc. all coa stuff.
So like you said there is something else that must play a role, not just haki. Imo it has to be something related to sword skills which Mihawk has been recognized even above Shanks and first in the verse as far as we know. That could be linked to boat (breath of all things) that Zoro has shown when fighting Mr 1 in Alabasta (as opposed to the voat of Luffy and even Momo).
So like you said there is something else that must play a role, not just haki. Imo it has to be something related to sword skills which Mihawk has been recognized even above Shanks and first in the verse as far as we know. That could be linked to boat (breath of all things) that Zoro has shown when fighting Mr 1 in Alabasta (as opposed to the voat of Luffy and even Momo).