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It sounds like she has allergies. Allergies can cause dogs to scratch excessively. The scratching damages the skin leading to a lowered immune response which allows the yeast and staph normally on the skin to multiply unchecked. This leads to skin dermatitis which could be yeast or a staph dermatitis. Staph usually occurs on the lower regions of your pet and tends to have small pimple type bumps. Shampoo containing Chlorhexiderm and/or Oatmeal can help with this condition though it does not cure the allergy. Yeast typically shows as a greasy area that has a sweet musty odor. Sometimes the skin can become inflamed, darker and thickened due to itching. Yeast likes areas such as between toes, armpits and ears. Selsun Blue Shampoo can help with Yeast dermatitis. When shampooing, lather and leave on 15 minutes before rinsing.
Once the skin darkens it usually stays that way. Since it is darkening, I suspect you still have a problem with a dermatitis. Instead of vasoline, you might try miconazole cream under the armpits. If this is a yeast dermatitis, this should help clear it up. It works well in areas where there is little hair. Another thing that can help once it is cleared up, is using benzoid peroxide wipes to keep areas such as the armpits and other skin folds clean.
Allergies can be hard to pin down especially since many dogs are allergic to multiple things and add the fact that if you happen to eliminate the source of the allergy (allergen), it may take 6 weeks or more to see improvement and any skin dermatitis present still needs to be cleared. I would suggest allergy testing so you know definitely what is causing the underlying issue. You can read more about allergies here:
And you can read about testing here:
Benadryl can also be given at a dosage of 1mg per pound every 8 hours to help relieve symptoms of allergies like itchiness and swelling.
If a staph infection is present, then antibiotics will likely be necessary so a vet visit is a good idea. A steroid might also help aid in healing if you have happened to have already eliminated the allergen.
Cushings can also cause darkening skin. Cushing's is when the adreanal glands overproduce glucocorticoid. This can be caused by a tumor, steroids and is mostly seen in older animals. Symptoms include excessive drinking and urination, increased appetite, increased panting, lethargy, hair loss, weakness, and a pot belly. Dog's with cushings sometimes have darkened skin.
Treatment and prognoses depends on the cause. In some dogs treatment is very effective. You can read about this here as well as more detailed information on cushings.
I hope this information is helpful and if you need clarification or more information, please reply.