Black blades are markedly different from adding armament. Zoro adding Koka is the first step to making an actual black blade. Not that his swords actually became Black from koka
Mihawk said any blade can become a black blade in reference to his black blade. Same as Tenguyama saying Zoro can make enma black. In fact Tenguyama says making a blade black increases it's rank so Unless you believe Zoro increases his swords to supreme grade every time he adds Koka, then Tenguyama and Mihawk are talking about other things
In fact Zoro even alludes to this. When Gyukimaru starts talking about black blades, Zoro gets surprised and asks what Gyukimaru is talking about with the whole "Ryuma forged his sword black"
Why would Zoro who knows Koka be asking about how black blades are made if he knows about it already