How do I treat a rash on my dog's stomach?

My dog is outdoors most of the day, comes in at night. During spring, summer and fall, she gets this irritation on her stomach. It starts out pink-red looking and by summer, her stomach is black. It itches, but usually clears up to a pink shade again by late fall. I'm thinking it's either heat rash, or maybe an allergy. Is there anything I can put on her stomach to relieve the itch?