how many battles will zoro have with enma before it becomes a black blade , whats actually stated is shusui became a black blade during ryuuma's many battles
its not stated as a defacto requirement, that doesnt tell you why it became a black blade. sounds like another assumption.
youve watched the character in a skirmish with commander level characters and youve built some fighter profile without seeing him use his weapon . How did aokiji fight in marineford , right , very different from the way he was actually trained, punching marine ships with garp to the point they were a physical match .