It sounds very much like a blackhead. These are quite common on the chin area of cats where plastic bowls and food clog up the pore but I have never heard of acne on cats belly. Other causes of blackheads include a hormonal imbalance which is something that should be investigated as changes in skin health can be an indicator of problems with the cats endocrine system. There are also conditions like mycetoma – which is a fungal infection that causes dark granular excretions from small lumps.