Neil, funny you should mention this. I'm about to selectively breed my pfr that have the black on them too.
Current theory is the black comes on as they get older.
I'm skeptical about that though. I'm actually thinking this is the way the now extinct Kanoko strain was created.
If you find something interesting, feel free to breed it out. Lots of hard work, space, money and tears- but well worth it emotionally if you succeed.