Everytime pre Wano King fight Zoro never had any black lightning being visualised when using CoA. It was always a aura around his swords and his swords and fists hardening. The first time when he showed black lightning was when he tamed enma and unlocked conquerors coating. So in zoro's case the black lightning he emits alsways stands for conquerors coating and not coa. He never showed black lightning in fishman island, dress rosa, zou or whatever arc he was in and used coa.