You have to wonder at the curiosity of ancient people finding the technique and the will to eat a black egg for the first time.

don't think.
your imagination is irrelevant.
you think too much.

It all happened like this

in long long ago times,
chinese of the teochew clan used to rear ducks at their backyard
which happens to be their open toilet too.

them were amazed some eggs collected tasted somewhat different
not good, but pungent and somewhat still edible.

later they found out those eggs were accidentally soaked in their urine

they eat those good eggs and sold those urine eggs to the market as medicine eggs.
surprisingly, the marketing plan worked.

thereafter they purposely soak those eggs in urine