Blue is a very popular color worldwide, and has been for centuries (the Crayola Crayon company did a recent poll on favorite colors; not only was blue number one, but of the top ten favorites, seven were shades of blue, and of the total fifty listed, sixteen were shades of blue). No one is sure exactly why, but psychologists and sociologists speculate that it may be related to the sky, which in ancient times was one of the only 'pure' colors in nature. I'm not sure I agree, but that's the speculation, anyway.
In the English langauge, the word blue traces back through Frankish and Germanic, to a proto-Indo-European word meaning 'light'. (As in light blue, not sunlight.) There are many color terms in other languages based on the same word - bhle-was - and not all of them mean blue. The word purple didn't exist in English at all until the 700s, so before that, the term blue usually meant what we today would call purple, also. (Something to keep in mind when we get to purple, next up.)
For much of human history, there was only one blue dye: Indigo. That's a block of it, there to the left, in it's half-processed form. Every continent has got some plant containing indigotin, the chemical that eventually becomes the blue pigment stuck to your denim jeans. Europe had woad, the Chinese had Dyer's Knotweed, the Maya had "azul Maya", the spanish term for the color (it bugs me that no one seems to know the Mayan name, when they're the ones who found it. Damned imperialists.) The Indian version of indigo was/is the most commercially viable because the plant produces the most indigotin. The name indigo is a bastardization of Greek, "From India". Even in the Classical era, Europe was importing it. Processing is complicated, involving a 'fermentation' after harvest, which is essentially putting it in water and leaving it to rot (much like linen production's early stages). After that, lye is added to the liquid, and the resulting gunk is formed into bricks and allowed to dry, producing something like the hunk in the photo. At that point it was transported/sold to wherever it needed to go, whereupon it was ground up, put into a huge vat of water and human urine, and allowed to sit until the oxygen was removed from the indigo and it became soluble in water. Then the fiber was dipped, and when removed from the dye pot, the indigo would re-oxidize and turn blue, much like an instant camera photo, only faster. (This must have seemed downright magical to the folks in the middle ages.) This process is one of the many reasons dye works were always located outside of towns, and downwind. It's very stinky. I've read that Elizabeth I of England passed a law that no one was allowed to manufacture indigo within five miles of her. It's good to be queen.
Unlike indigo, which is a fairly stable dye, blue paint was a bit trickier to produce. Especially a colorfast paint that didn't cost a fortune. The first synthetic paint was blue, produced by the ancient Egyptians at least as far back as 3000 BCE. They cooked up what was, essentially, blue glass in a kiln, with quartz sand, natron, copper, and other odds and ends, then ground the resulting glass and mixed it with a binder. Voila. Blue paint, known these days as Egyptian Blue - see photo at top, left. It was also used as a ceramic glaze, in which case it is called faience. The Chinese did something similar, from about 500 BCE onward. The color is from a blue glass, and is called Han Blue, or Han Purple (see above, about how blue and purple were all blue until the middle ages). Among other things, it was used to paint the terracotta warriors in the Qin tomb at Xi'an - photo at center left. The Mayan indigo was transformed into a synthetic paint with the addition of clays and became one of the most stable blue paints of the ancient world - photo, bottom left. It has been speculated that the bronze age was born while early chemists were trying to make blue glass - the colorant in all these paints is copper, and bronze contains copper and tin or arsenic. Not sure if it was the chicken or the egg, but either way, I bet the two processes were related.
In the middle ages, it's said that the primary colors were vermillion (red from mercury sulfide, cooked up by alchemists), gold leaf (yellow, made by pounding the hell out of gold coins), and ultramarine, a blue so expensive that the gold leaf was more affordable. All those medieval religious paintings with big swaths of rich blue, on Mary Magdalene's robes and the blue skies? A form of major conspicuous consumption by the Church (or whoever else was paying for the painting). The cost was so high, that when contracts were made between painter and patron, it was very often speicified which pigments to use, and often the patron was expected to procure the ultramarine himself, and then pass it along to the artist. It wasn't uncommon for the patron (or a minion of the patron) to stand over the artist while he was painting the blue bits, to make sure the ultramarine wasn't wasted. Ultramarine means, literally, 'over the sea', and the fact that it's imported is only part of the reason for the high cost. The other reason was an insane manufacturing process: Lapis lazuli from Afghanistan (the mines are in the middle of nowhere, even by Afghan standards, which means major transportation costs passed on, ultimately, to the patron) was ground to a powder. The powder was mixed with various resins and bees' wax, and kneaded off an on indefinitely (directions from the time say at least a week). Then you put the mess into a cloth bag and chucked the whole thing into a bowl of lye, and mooshed the gunk around with two sticks. Lazurite, the blue stone in lapis lazuli, would drift to the bottom of the bowl. Then the lye was allowed to evaporate over several weeks, the resulting crust was ground up, and mixed with binders into paint. Until a process was developed in the 1820s to make a synthetic version (which is chemically the same but cooked up in a blast furnace), the paint remained insanely expensive. A less expensive substitution was ground azurite, which is pretty much the blue version of malachite, and handled the same way.
Eventually indigo and ultramarine were synthesized, putting a lot of Indian farmers and Afghan miners out of work, and helping kick-start the synthetic dye industry, and most of industrial chemistry as we know it today. International Klein Blue is the first color developed for a specific artist, by a chemist, working as a team. It was 1958, and it kicked off a whole new approach to color that we're dealing with today, where designers and artists can request colors and have them tailored to exact specifications. All it takes is deep pockets. Ah, if only Titian were alive now. He'd wet his pants with joy.