Does anyone know what these dark spots are?

My dog has developed these dark patches on his groin and armpits. I have no idea what this is from. We have moved recently to a new area about 3 months ago and I have been calling around to find a veterinarian. Most of them I’ve called said they are booked until late March or early April. My dog Phil seems itchy, but other than that really normal. The skin is not raised or broken, but I can tell he seems bothered by this. I’ve been feeding him the same food, NutriSource duck and quail brand for over 3 years. It’s slowly and progressively spread, I plan to take our chances on a walk in appointment on Sunday morning. Has anyone ever experienced this?