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You mention that your dog's ears are crusty, Usually when a client mentions crusty, it is a discharge from the ear. A black discharge could indicate an ear infection or mites. Mites, however, are not common in adult dogs. You can read about them here:
It is probably a yeast or bacterial infection. You can use a mixture of distilled vinegar, water and alcohol on a cotton ball to gently clean the ear
In a yeast infection, you will see a golden to black buildup in the ear. You may also notice redness and bleeding, head shaking and your dog scratching the ear as well. You can try some miconazole cream to see if it helps until you can get in to see the Vet, but it isn't a cure for underlying condition
You will have to have your vet look at it and determine if it is fungal or bacterial in nature so the appropriate medication can be given. Ear infections are not pleasant for your dog and I hope you can get him seen soon.
The following site has a recipe for a home remedy that may provide some relief though I recommend visiting the vet for medication.
Zymox with cortisone has been used by some experts to clear ear infections as well. You can read about it here:
It sounds like an ear infection but in some cases, ear infections are the result of an allergy. So I really can't tell you that the ear infection was the result of an allergy or just developed on its own.
I hope this information is helpful to you.