I noticed the same just Sunday on one of my girls by her backside vent......the biggest one! But obviously lower in the pecking order. I thought at first mange? But then I saw the problem hen doing the feather plucking. I immediately thought the same on protein deficiency, so they have had a supplement of scrambled eggs and mealworms, but then I read your post and maybe not. They are on organic grain and love it of appropriate protein for layers with calcium supplement, will consider boredom and of course eliminating the bully. Hope she is only one doing it. Is the blue treatment you mentioned the best?