To me it looks like poo that she'd gotten into. Is it in between the scales (like it looks from the picture) or is on the scales?
Did you use a damp cloth to try to remove it, or just your finger?
It doesn't look like a thermal burn to me, really. But I'll ask none the less, is there anyway for her to get her head hot? Or anything that she could have scraped her head on?
If it doesn't appear to get worse, see if it passes with her next shed. If all else fails take her to the vet.
Sorry I couldn't be of more help.