Sounds like seasonal alopecia. You don't state what breed your dog is, however this is a common condition in many breeds particularly in bull dogs.
It starts off as a small patch which looks to be thinning, the area gets bigger and becomes bald and the skin underneath turns very dark - almost black in some.
This condition is harmless in itself and unless it is causing irritation, treatment is for cosmetic purposes only. It is commonly found alongside the flank of the dog, and often is bilateral too.
Often, the hair loss is seasonal/hormonal and will grow back itself.
It starts off as a small patch which looks to be thinning, the area gets bigger and becomes bald and the skin underneath turns very dark - almost black in some.
This condition is harmless in itself and unless it is causing irritation, treatment is for cosmetic purposes only. It is commonly found alongside the flank of the dog, and often is bilateral too.
Often, the hair loss is seasonal/hormonal and will grow back itself.
