Black spots can be quite normal, most especially in dogs who have any sort of allergies or inflamed skin. My Wylie has had them forever.
As for your Mom's dog, I *highly* doubt it's a food allergy as they're actually very, very rare -- while enviro allergies (pollen, dust etc) are VERY common. Have your Mom read this thread:
http://www.yorkietalk.com/forums/die...tart-here.html