Blue is a primary colour and also the only colour that is represented both in CMYK and RGB colour models. Before we were able to create blue pigments chemically, it was quite troublesome to get a nice blue. Mineral pigments came from the semi-precious stone lapis lazuli and were extremely expensive. Natural dyes came from woad leaves, but took a long time to create and washed off quickly. Only later, when true indigo was imported to Europe from Asia, did the production of blue dyes improve significantly.
In the 9th century, Chinese artisans found a way to use cobalt pigments to produce beautiful blue porcelain, which caused a sensation in Europe. Blue and white china quickly became popular and all countries found a way to make it their own. Here is a traditional Bohemian pattern lovingly nicknamed the blue onion.