This is the first egg of the season from one on my gals (khaki campbell egg above it)
This pic shows how the black is simply a coating on a lighter colored egg. You can see how a simply scratch on the shell will remove it!
The green eggs that have a hint of off color on them are also cayuga eggs. They too started out black and slowly with the egg cycle less and less black appear on the eggs
Another example of how they fade. A fairly early season egg at the bottom on the left. All other eggs are cayuga eggs but later in the season. Most are white, some stay "purple" or a lighter color gray
3 shades of cayuga eggs. Black, white, and "purple"