Ahhhh see this threw me - I didn't realise that was normal!I’ve hatched almost 50 BCMs chicks, so far all of them has white on chests and under wings.
Yep, I got one today as I was so excited to see some copper highlights coming through (I'd convinced myself it was just black!).Can you take close up photos of your Marans maybe I can give some hints?

I'll try to get some better ones of him/her tomorrow!