Not sure about the dark dogs but itchy ears/chin/paws is often related to food allergies. When the armpits are included I would wonder about a systemic yeast issue.
After switching off a a food that you think could be the culprit, keep your dog on the new food for at least 60 days to see if there is a difference.
You can test for environmental allergies but food allergy testing isn't reliable.
One theory on why so many dogs are allergic to chicken and it is in most dog foods is this. They have been fed too much of it. In other words, if all dog food contained lamb we would see an increase in lamb allergies. If a dog doesn't have constant exposure to one particular food, we may see less allergies.
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