The causes of black protruding goosebumps under the armpit include folliculitis, keratosis pilaris, nevus, melanoma and so on. 1. Folliculitis: If the armpit is infected by bacteria, the bacteria will damage the hair follicles, resulting in black skin and swollen hair follicles, so black goose bumps appear. 2. Keratosis Pilaris: The skin around the pores produces excessive keratin due to heredity and other factors, which protrudes out of the skin like black goose bumps. 3. Moles: Due to external environment, immunity and other factors, melanocyte proliferation in the armpit skin can form black bumps after protruding from the skin. 4. Melanoma: Because of many factors, melanocytes proliferate and become malignant, so black bumps will also appear. If you have black pimples under the armpit, please go to the hospital for examination, clarify the cause of the disease, and under the guidance of the doctor for treatment.