It's possible that she might have mites. I'd say schedule another vet appointment and let them check her over.
You can also try using a black shirt and rubbing it down her spines, then see if any bits come off and check if anything's moving. If you don't see anything moving, don't rule out mites! You could have just not gotten any of them on the shirt.
Also, the dark spot, if it's not a lumpy or raised in anyway it might just be mottling.