As I pointed out sometime, I think that Breath of all things (BOAT) is advanced CoA + CoO. CoA for cutting everything, CoO to hear the breath (feel the presence) even of non living things.
So I think that Zoro will soon turn Enma black, indicating a very advanced form of CoA.
While against Shiryu, he will learn to control that advanced form of CoO, which will allow him to hear the breath of Shiryu's body while he is invisible.
Your thoughts?