Enma doesn't have to become black for ZKK to happen. But both ZKK and blackening Enma are connected to Zoro fulfilling Oden's unfinished will. If Enma had been forged black King, ZKK would have been less likely.
I think Enma turns black after the Ryuma parallel moment.
Don't reply to me if you're only going to offer this kind of response.
It's an especially shitty ad hominem.