I have 9 hens and I have seen them all in nest boxes except this one. By process of elimination I think she lays a dark green/slightly olive egg which makes me think she is a mix. She is black with blue opalecent color the feathers and black legs. Black and white neck and white ear. The last picture is when she was a pullet. If you google blue birchen marans there are pics of chickens that look like her but seems like they are dark brown egg layers. I think she lays at night or at the crack of dawn so I can never catch her. The mystery continues .