I Have A Pitbull/lab Mix And She Is Getting Dark Patches Under Balding Spots On Her Back Inner Thighs... What Could It Be Caused By? Or What Could It Be?

This could be ringworm, mites, or a secondary response to allergies.  The itching and scratching secondary to allergies can lead to hair loss, redness, yeast and/or bacterial infections, and pigment change.  The inner thighs are a common place for allergies. 
I would have your dog examined by a veterinarian to determine the cause of the hair loss and hyperpigmentation.