It looks like some sort of injury that is healing up. I purchased a proven female a few years back that has a scar like this on her back as well. It has never impacted her one single bit - to the point that she even gave me 11 good eggs last season. Here she is today:
Just keep an eye on it. If she takes a turn for the worse (it looks raw/infected/etc), then seek a vet's advice. If not, then I'm sure she'll live a nice happy life like any other BP would.
Best regards,
Eric
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