What is your point. The sword is still white and blackif you are assuming the sword is coated the prove it. We have seen Enma coated before, just compare both images
And you're right. We have seen Haki used on Enma before, and it looks exactly like the panel of Zoro holding Enma after attacking Kaido. It's black on the back edge, black on the sharp edge, and white straight down the middle.
Zoro used Haki...