See if she is up for a shower.
Might come off then?
No chance of the food causing a stain? (Example that comes to mind is
Diospyros digyna - Wikipedia, the free encyclopedia - or Black Sapote; while I've never heard of parrots eating it, I know when we, human's do, it causes very awkward finger and mouth stains.
)
some birds can get this if they are trying to reach food they have dropped between the wires at the bottom of the cage, or try to shred the paper there, and can get dirty faces from that... have you seen her doing anything like that possibly?
Based on what Oedipussrex has said, go through and clean your entire cage. Get a cloth and run it down each individual bar in the cage. If it's coated, there could be an area where the the powdercoat/plastic coating has flaked off the cage and is rusting.
The other possibility is it could be an allergic reaction. I've heard of some cases where the white patch of skin can get irritated and then red bumps/rashes appear. (But it doesn't appear to be red in this case)
OR perhaps a feather shaft got broken and that's dried/dead blood. (It can dry as black or brown in some cases)
If you're not able to find a cause, a trip to the vet may be worthwhile
Remember that birds hide their illnesses very well.
