What Could Dry Rashes On Knee In Children Suggest?

Hi, My son is eight years old, he has got beside his knee black rash, it looks, as if boil water or oil spills it was looking like a boil, Initially i thought boil water would have splashed, but now it is black flat rash. This rash is dry and itches for him, and nearby the rash small black rashes also started appearing, i consulted the doctor they gave him a cream ( iam sorry unable to remember the name of the cream) and every night one small table he has to have, after having this tablet and applying the cream, no changes, what could be the reason plz help