I have one...quite by accident this happened before I sold my cuckoo flock. I do believe it was a cuckoo roo over a black copper hen.
My pullet turned out to be solid black with a slightly wheatish ring, not quite or nearly as prominent as the red round a black copper marans hen's neck.
Her egg is darker than buff and shiny, but definitely not to the lowest standard of either a cuckoo or copper marans egg. She's out ranging now. I'll see if I can catch a pic of her...