I had this happening a year or more ago in Blue Orpingtons!! Do not use them for breeding purposes. I would cull (and that doesn't mean you have to kill them off) from the flock. It happens sometimes in black birds is what I was told! Mine were bbs orps so that meant blue, black or splash colorings.
Someone with more experience with genetics hopefully will chime in.
Edited to add it only happened in acouple of my roosters not any of the hens.