For a long time in history, there was little blue to be seen. The Ancient Greek didn't even have a word for it. On maps, the seas were often pictured green. So when at the end of the Middle Ages, a truly bright blue stone became available in Europe, it changed art in dramatic ways. Called 'lapis-lazuli', the stone was made into the paint 'ultramarine', which means 'from across the seas', since it came from Afghanistan. Before this, blue was almost never used in art history. Soon the Church sought to control, so that it was only used for the most divine parts in paintings. For a time, it became even more expensive than gold.
In the Romantic Age, blue became known as the colour of our deepest feelings. It remained this way when the impressionists repeatedly called for blue when they tapped into their deepest emotions. Later, Picasso chooses this colour during the time he mourned the suicide of his best friend, known as his blue period. Yves Klein even made his own blue, the colour being the art itself. He wanted to leave this world behind, with blue representing the infinite. Just how looking to the sky or sea evokes infinity. Decades later Dutch artist Daan van Golden uses blue for the same reasons.
Blue has been the colour of the world beyond our own. But in 1968, when, for the first time, we were able to see what the Earth looked like from outer space, we learned that blue was actually the colour of our home. Still, because of its long history of use, blue continues a colour that artists choose when visualizing something of our imagination, out of reach, at a distance or divine.