Zoro still doesn't even understand, and has very limited information on Black Blades. Even when he fought Onimaru, and tried to ask about it, Oda just brushed it off. It would seem really out of place if Zoro just Blackened his blade out of the blue, when Oda has been very descriptive as of late when handing out powerups to his characters.
What is Zoro going to come to realize about blackening a blade that will allow him to do it, when he barely knows anything about Black Blades? He needs to at least have some kind of familiarity with it, in order for it to happen in a believable way.
Is he going to come across someone from now until the end of this arc who broadens his knowledge on the matter?
There hasn't really been any setup for it at all.