Yes, it’s just oily discharge from the glands in the skin. And then it gets dirty so it looks dark like that. Some gentle scrubbing with a wash cloth with warm water and baby shampoo should get it off.
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.