There arent any scales missing or any physical damage. Might just be the pic looking like that. It looks and feels just like the rest of her body just looks black right there.
Probably just a little bruise. She may have jabbed herself on a little branch or something moving around and now you're seeing the bruise. They seem to bruise easily sometimes. Keep an eye on it anyway, but I'm willing to bet that it'll go away soon enough.