I wish I had some pics of feeder guppies to show you, and I didn't have much luck finding any. Sounds like you have females. And if they have a black spot in the abdomen, it sounds like they are already pregnant. In my experience, even with feeders, the males will have some color as they mature. Usually orange or black. Given time, it will be obvious which is which. The males will display and the females will be bigger. But, like TankGirl said, feeders aren't quality fish and you may need to go through quite a few to get some good healthy ones to keep for breeders.