Browning bar safari, stain on blue question

[godgab]

Hi there browning owner, I just bought a browning bar safari mark 2 used and after inspecting it I saw a small stain on the receiver bluing, I though it was a sticker that was there or something but then I saw that there's a similar stain on the inside of the receiver. I was wondering if it was because this part was heat treated or something like this. I emailed browning service and I'm waiting for an answer.
Here's the pics, tell me what you think.

https://ibb.co/bBYtBK
https://ibb.co/h2ibWK

thanks