"Ultramarine was made from lapis lazuli, from the mines of Badakshan, in the mountains of Afghanistan, near the source of the Oxus River. The mines were visited by Marco Polo in about 1271; he reported, "here is found a high mountain from which they extract the finest and most beautiful of blues." Ground lapis was used in Byzantine manuscripts as early as the 6th century, but it was impure and varied greatly in colour. Ultramarine refined out the impurities through a long and difficult process, creating a rich and deep blue. It was called bleu outremer in French and blu oltremare in Italian, since it came from the other side of the sea. It cost far more than any other colour, and it became the luxury colour for the Kings and Princes of Europe.
King Louis IX of France, better known as Saint Louis (1214–1270), became the first king of France to regularly dress in blue. This was copied by other nobles. Paintings of the mythical King Arthur began to show him dressed in blue. The coat of arms of the kings of France became an azure or light blue shield, sprinkled with golden fleur-de-lis or lilies. Blue had come from obscurity to become the royal colour."

https://en.wikipedia.org/wiki/Isatis_tinctoria
"German indigo, of the plant family Brassicaceae, have been found on pottery in the Iron Age settlement of Heuneburg, Germany.(close to the modern borders with Switzerland and Austria. It is considered to be one of the most important early Celtic centres in Central Europe.)" "the earliest light-pigmented and blue-eyed remains of Homo sapiens were found in 7,700 years old Mesolithic hunter-gatherers from Motala, Sweden."

https://en.wikipedia.org/wiki/Prussian_blue
"Another name for the color is Berlin blue or, in painting, Parisian or Paris blue. When Brandenburg became the Kingdom of Prussia in 1701, the uniform colour was adopted by the Prussian army. Most German soldiers wore dark blue uniforms until the First World War, with the exception of the Bavarians, who wore light blue.British naval officers simply wore upper-class civilian clothing and wigs. In 1748, the British uniform for naval officers was officially established as an embroidered coat of the colour then called marine blue, now known as navy blue."

"George Washington organised a voluntary militia unit (the Fairfax County Independent Company of Volunteers) and elected Washington the honorary commander. For their uniforms they chose blue and buff, the colours of the Whig Party, the opposition party in England, whose policies were supported by George Washington and many other patriots in the American colonies."
"Napoleon Bonaparte abandoned many of the doctrines of the French Revolution but he kept blue as the uniform colour for his army, although he had great difficulty obtaining the blue dye, since the British controlled the seas and blocked the importation of indigo to France. Napoleon was forced to dye uniforms with woad, which had an inferior blue colour."
"Blue was the colour of liberty and revolution in the 18th century, but in the 19th it increasingly became the colour of government authority, the uniform colour of policemen and other public servants.....officially known as NATO blue"
"In the 21st century, the dark blue business suit is the most common style worn by world leaders, seen here at the 2011 G-20 Summit in Cannes, France."
"Following the Second World War, blue was adopted as the colour of important international organisations, including the United Nations, the Council of Europe, UNESCO, the European Union, and NATO."
"With the dawn of the World Wide Web, blue has become the standard colour for hyperlinks in graphic browsers"

https://www.unicef.org/press-releases/iconic-landmarks-around-world-turn-blue-symbolic-stand-child-rights-world-childrens