I had similar black streaks on my roof and hood. I think it is glue residue from shrink wrapping these cars had during shipping. I would wash and they would not come out or reappear after couple days. Then I just took a little wax and cleaned the spots with it. They totally came out and never reappeared. Clay bar should only be used to remove embedded very fine particles in paint. Since it removes wax as well as contaminants a good wax and seal should follow any clay treatment.
If wax works by all means do that first, always use the least aggressive method first. I just figured maybe since they washed it they might have tried awax already, and in my experience from formerly working at a dealership sometimes the glue can be really stubborn and a clay bar was sometimes the best/easiest way without causing any marring of the paint. That said, definitely don't use clay unless you need it 🙂
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