What causes hyperpigmentation? 

Skin gets its color from a substance called melanin, which is made by skin cells. When those skin cells are damaged or unhealthy, they can produce too much melanin. The melanin can clump, causing that area to appear darker.

Skin damage from acne, blisters, cuts, sun exposure, genetic factors, and autoimmune conditions are all possible causes for changes in melanin and, thus, skin pigmentation.