There are a lot of different blues: cobalt, Egyptian, Prussian, and ultramarine, to name a few. But Indigo blue has the best story.
Indigo has been used in India for more than 5,000 years ago. In the 1700s, it was in demand because it made a much better, more intense blue than Woad Blue. Traditionally, Woad and Indigo were processed by soaking the leaves of the Indigo plant for a week in human urine, then putting the fabric out in the sun where as it dried it turned blue. Entire towns saved their urine for the production of indigo dye!
The English had a monopoly on the Indigo dye from India and it was very difficult to get at any cost. Indigo was a better, more intense blue, and people had paid a premium for the blocks of dye that came from a long ship voyage.
The dye got a huge boost when a young Eliza Lucas planted indigo seeds in 1739 on a farm in South Carolina. She convinced her neighbors to grow Indigo and before long, the Americans had cornered the market for Indigo blue. (By this time, the Americans had developed a way to process the indigo without urine!)
Indigo became the second most important cash crop in the South Carolina(after rice) before the American Revolution. It comprised more than one-third of all exports in value. The British responded by growing Indigo on plantations on the Caribbean islands.
Thanks in part to the availability of indigo dye, military uniforms were blue. Napoleon could not get indigo because the British blocked the importation of indigo to France, so his army had to wear uniforms died with Woad, which had an inferior blue color. Synthetic Indigo was not developed until 1890.