Hello, I'm a small animal veterinarian and will try to help answer your question about Beauty.
Yes, these black specs are likely to be mite feces.
The mites themselves would be fleshy colored - have you thumbed through his feathers to see if you can visualize mites.
I would suggest a visit with your local avian veterinarian for diagnosis/treatment. Bring Beauty's regular cage to the visit if possible (i.e. it's not too large)
This will allow your vet to look in the cage crevices where mites can hide.
Typically we need to treat these guys with ivermectin
The store bought mite sprays/etc don't work. Hopefully this helps
Please feel free to respond