My dog's belly has black specs after a rash. What should I do?

I'm not quite sure what it is, either! It could be small amounts of dried blood in the pores, dirt, or debris from the bumps healing. I can see a few more pustules in the second picture, so it looks like Panda hasn't completely resolved the original issue. You can try a follicle flushing shampoo, like Dechra DermaBenSs Shampoo which contains benzoyl peroxide, but Panda may need oral antibiotics to clear up any infection present. Also, make sure she is on a good flea preventative as sometimes flea bites can present this way. If it isn't looking better in a couple days, Panda should definitely go to the vet for diagnosis.