Blue is a colour associated with prestige, harmony, clarity and curiously, both happiness and sadness. Surveys tell us it's the most popular colour in western culture, but in other cultures it can indicate mourning or villainy. It's currently considered the colour which typifies boys but in earlier time was considered more appropriate for girls. It's the traditional colour of jeans, the most universally recognizable item of clothing. It's used by some animals to attract mates and by others to ward off predators. And it's the colour of the sky.
On a clear day it's everywhere we look. So rich and untouchable, its hues are unmatched by anything in our terrestrial experience. The shade of blue may vary according to the season and time of day, but as a sign of fair weather and fair winds, a blue sky is unambiguous in any culture and era.