As the fleas are gone, the scratching will stop and the rash will fade away on it's own.
Unless it is bloody and looks infected that is.
It's critical to get all fleas gone, use a topical flea med like advantage 11, which will also kill the eggs and larvae so reinfestation doesn't occur.
You can sponge witch hazel on it a couple times a day, this helps ease the itchiness and cleans the area nicely.