I've seen these on my dogs in two situations. The first is as Halia said - just dirt clinging to oily areas. The second is a skin infection. I would give her a good bath, and use a wash cloth or something to see if you can get it all off her tummy. If it comes back, spreads, won't come off, or seems to be causing her discomfort then it wouldn't hurt to let your vet take a look.