Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
How long have these been present?
Did it start before the diet switch or after?
Are the black spot raised or patches on her skin? Or are they something you can remove or wipe off?
Any crusting, scaling, pimples, sores, or pustules?
Can you take a photo of this? If you can do so, this will let me see what you are seeing. To post them online, you can use the paperclip icon on the tool bar above your text box. Or if you can't see that on your computer/phone, then you can post them on any other site (ie Flickr, Photobucket, imgur, etc) and paste the web link here for me to check.