I do not have a Cayuga, but I do have a duck with black feathers, and mine got the same iridescent blue/purple/green color around wings and head, and everyone here told me it was normal.
My own experience over time has confirmed it. I think it's one of the glories of having beautiful black birds!!
Yes, cayugas have a purple/blue/green sheen to the feathers.Sunlight effects the coloration.If your duck is for showing they recommend limited sunlight time.Mine are just my pets...who lay eggs and entertain me.I love the coloration! Beautiful duck