Scale Rot? Or something else?

[StephenT]

What did the guy you bought him from use as substrate ? Because that looks like nothing to me so much as ink from newspaper, maybe rubbing off there because that would what gets damp/wet when the snake drinks.

It certainly doesn't look like scale rot.

It does look a lot like ink, @OP try giving your snake a good wash down. I'd avoid soap since I'm not 100% sure it's snake safe, warm water and a mild scrubbing should suffice.