A single mutation in an individual probably living in the region north-west of the Black Sea is now thought to be the origin of blue eyes. According to scientists in Denmark, the trait then spread with the large migrations of people seeking new agricultural land at the end of the last Ice Age.
Quite why the trait has survived and appears to have flourished is not known, however. It may have offered a survival advantage in the cloudy and darker north or it may have enhanced sexual attractiveness. In ancient societies men would have been in short supply anyway as many would be killed either in hunting expeditions or skirmishes with other groups. Anything which gave a female an advantage in mating with the few available men is thus likely to have had a real effect in spreading the gene into future generations.
Another article on this study. I'm beginning to have my doubts about some of it. Especially the very recent mutation of only 8,000 yrs. ago.