Oxydation spots inside stainless steel flask

Hey y’all.

Bought a flask, used it 2 times. Always cleaned it with scalding hot water and let it air with the cap off when not using.

I look inside with a flashlight and I see black spots on the steel, with a q tip i was able to wipe it off, leaving black smudges on it. Oxydation ?

Anyways, is it safe to use ? I couldn’t manage to clean it all off even with hot water treatment. I’m hesitant to use it now.