“Some of the skin around his tummy was all black!”
Below is an excerpt from: http://www.allergydogcentral.com/tag/hyperpigmentation/
Some allergic dogs also have issues with dark, almost black patches appearing on their skin. This is known as hyperpigmentation, a condition in which patches of skin become darker in color than the normal surrounding skin. Hyperpigmentation is often combined with hair loss or balding.
As with all allergy symptoms, if you see reddish discoloration or signs of hyperpigmentation, you should talk to your veterinarian. They should be able to help you to determine if your dog is indeed suffering from allergies, or if their skin condition is related to a different health issue.
