Chapter 779
Page 3
Mihawk (talking to Zoro in the flashback) : Have you ever broken a blade?
Zoro : A few times. You did It to me, even!!
Mihawk : If you had infused them with Haki, those swords would not have broken... You must treat every nick as a mark of shame. Every weapon can be turned into a black blade like mine. No liquor until you have mastered this feat.
So if he were using it, he sure didn't know about it and it was a very initial phase and stage of Haki.
I believe he used Kenbunshoku tho when telling where his swords were, vs Mr. 1. That seemed like the state Coby was in too, when he awakened it at the war.