My fur baby Zoey had a dark spot in the same spot your baby's black spot is a few months ago. Zoey had loss hair in the area and I had no idea how she had gotten it. We took her to the vet and the vet had made the assumption that Zoey may have burned herself somehow and we were given cream to help aide her skin recover (the black spot looked as if it were a scab & was already flaking off during the vet visit). To this day, the hair hasn't grown back
I hope you were able to take your baby to the vet to get it checked up!