The darkening of the French Bulldog's belly may be caused by ultraviolet radiation or pigmentation, which is a normal phenomenon and generally does not affect the dog's health. It is usually best not to let the dog lie directly on the ground. You can prepare a small mat for the dog. This can reduce the friction between the belly and the ground and reduce the pigmentation. In addition, the dog should be bathed and dewormed on time to avoid skin infection. The disease causes the dog's skin to turn dark.