Short exposures to sun of aroud 5min per day are actually good for the body, since that's how we generate vitamin D. It's a major antioxidant and it's used in bone formation. But aything more then that and it's excessive.
As for dark skin it acts like sunscreen. I've read something ages ago (don't quote me on that) that the skin of a person with really dark black skin would be equivalent to SPF rating of 4 to 8. I can't remember the exact number, but it was quiet low. So if you are going to be out in the sun for long periods of time, put on sunscreen. It's especially true where I live since the ozon is so low, that you can get a bad sun burn in less then 5-10min.
As for dark skin it acts like sunscreen. I've read something ages ago (don't quote me on that) that the skin of a person with really dark black skin would be equivalent to SPF rating of 4 to 8. I can't remember the exact number, but it was quiet low. So if you are going to be out in the sun for long periods of time, put on sunscreen. It's especially true where I live since the ozon is so low, that you can get a bad sun burn in less then 5-10min.