Acne can occur in any skin type. But because of its link to hyperpigmentation, it is of greatest concern for people with dark skin. Acne forms when there is an overproduction of oil in your skin. That oil mixes with bacteria within the pores and blocks the pore openings. This causes inflammation just under your skin, which leads to lesions -- anything from small bumps to large cysts.
Not only can this lead to pigmentation problems in dark skin, but so can many medications used to treat acne. This is also true of certain oral antibiotics, particularly minocycline. In some people, it can darken the skin. This can take months to fade.
If you have dark skin and get acne, see a dermatologist skilled in treating ethnic complexions as soon as possible. The earlier you seek treatment, the less likely you are to suffer permanent pigmentation scars.