It's the 8 hours thing I'm surprised by. And I assume the fish wasn't checked in between so this probably happened in less time. And black going to total white...I'm no Koi expert but I've never heard of that either. In Goldfish yes, but over weeks.
The only creatures I've heard of that can do this and still be alive are those with chromatophore cells. If you put him into a blue box and he turns blue I'll buy him from you
Which Koi did this happen to?
I assume this isn't a massive fungus infection because that would be obvious. And I assume the Stress Coat didn't react with something and go opaque. I mean the fish looks like a normal all white fish? You can see and feel the scales? Is this even a scaled Koi?
If you really wanted to you could treat with salt just to see if the mucus pushed off some opaque coating or something under the scales. But I don't know.
It'll will be very interesting to hear how this plays out. Hope you keep us posted Darryl.