>>33762585Melanin in the body has a protective effect against UV rays in harsh sunlight. The higher the melanin content, the darker the skin. Sunlight is composed of UV radiation which causes DNA damage to the skin (cancer) but also helps the body produce Vitamin D, which is necessary for good bone health among other things.
In warm climates near the Equator, there is higher UV radiation than places closer to the poles. Melanin is therefore necessary to prevent radiation damage, so people that originate from here have high amounts of melanin and are dark skinned. Vitamin D is not as affected due to the high levels of sunlight that hit the skin anyway.
However, since melanin blocks the effect of sunlight, in places with a lot less sunlight this can cause Vitamin D deficiencies. Therefore, the people that originate closer to the poles have evolved over time to have less melanin and therefore lighter skin, meaning that they are more likely to get sunburned or suffer skin cancer if they are exposed to large amounts of sunlight without adequate skin protection.
As for why these people are blue, it was probably to identify them as people from another dimension, compared to just pale people in the regular world, as well as alluding to them not being exposed to much sunlight.
