Either Oda retconned Shodai into being a Kokuto, or Venus is trying to break the world record for keeping his blade hardened the longest. His blade has been drawn as nothing but black this entire arc so far. If it’s drawn like a normal sword, I’ll take my L, but it makes sense that Oda would retcon Zoro’s potential final opponent into having a Kokuto.
We have seen Hyogoro and Rayleigh not even turning their blade black while using Advance Haki, and alt @Foul Legacy clown denying that he wasn't debunked and they didn't use CoA Haki.