Round black scab on whippet

The other day I noticed a small lump on my dogs neck. There is a hairless circle around it and a central black sort of scab. She's only 2 and I think very healthy but I'm imagining the worst. We are taking her to the vets but from an initial picture they didn't know what it was. She has not had any injury to that area and she cannot scratch it because of where it is at the back of her neck/top of her back Any ideas?