Black blades will always remain unanswered just like Advenecend CoC. Just like Luffy at one point just started using advanced CoC with zero explanations as to how it works, what's the difference of CoC and CoA etc. In the same manner in one of his epic fights Zoro's blade will turn into black with zero explanation as to why and how. How many times does it have to be repeated for you people to understand that stuff like that WILL NEVER EVER, UNDER NO SIRCOMSTACES BE ANSWERED. Because Oda or his staff have never thought it out, they never focused on a cohesive power structure, it's abstract, it's just in your imagination, Oda never explained and never will. Deal with it!