In this case, you would say "She's tanned" (or she has a tan, she has a tanned complexion.) In the US, somebody "black" would be an outdated description of a person with African roots. By the way, "she has dark skin" or "dark complexion" would indicate someone with Mediterranean, Latin American or African origins.

Skin does not get black from outdoor activities. It gets tanned, as the other reviewers stated.

You should say she's tanned or brown skin (like me) =D