Skin color is genetically determined.
From the dermatological point of view, we classify skin into six types. Base on the susceptibility to sun burn and the readiness to develop sun tan, those who always burn and never tan are type I skin. While those who are darkly pigmented and never burn are type VI. Most of the ethnic Chinese are in-between these two ends and are type IV skin. The fair skin in Caucasians belongs to type II or type III skin.
Skin complexion is actually a combination of the result of skin thickness, skin blood supply, and most importantly the pigment deep in the skin. Our body pigment calls melanin. The brown-black pigment called eumelanin and the yellow-red pigment called pheomelanin. And the cells that manufacture melanin are residing in the lowest layer of the skin. Ultraviolet light, particularly the UVA spectrum stimulates these cells to produce more melanin. So we get darker after working a long period under the sun. Melanin not only contributes to our skin, pupil and hair color, but is also essential to protect us from the damage of ultraviolet radiation. It explains why people with darker skin color are less common to develop freckles and skin cancers.
Neither too many nor too little pigment producing cells is good to our health. Albinism and vitiligo are the diseases in which pigment producing cells are absent. Dark mole is caused by having too many of these cells cluster together. When they behave badly, as you note the mole is getting larger or darker, it is likely transforming into a cancer.