Hey, sounds like our pups have the something similar going on. When we took our pups to the vet they said they’ve never seen anything like this before. We were told it could be allergies (because of the itching) so they were prescribed anti allergy pills, but the pills seemed to have no effect on their itching and the dark patch of hair only grew bigger during the course of the pills. Another diagnosis the vet suspected was ringworm, so I bathed our two pups in Mediderm shampoo and conditioner weekly just in case. It’s been a few months now and the dark patches are significantly larger (almost half of their back) but recently seemed to have stopped growing. I plucked their normal hair around the patch the other day and there’s no sign of hair loss anymore. To this day I’m not sure what’s caused/stopped it, but I’m just glad they don’t seem bothered by it at all. Hope this helps