I think the better way to understand this is to know that the melanin difference between the fairest skinned person and the darkest skinned person is way smaller than you think. Something like 2x. It makes a big difference to our eyes, but it makes almost no difference to deadly radiation.
Imagine your skin magically became twice as thick, twice as tough. This would be convenient to avoid minor scrapes and scratches, but if you get stabbed or shot you will die just like anybody else. Hence, darker skinned people fare better when fighting off sunburn, but their skin melts off just like anybody else's when the rad levels get truly dangerous, with basically no difference at all.
exohuman