The teacher of Raphael was a highly regarded artist named Perugino. Perugino was one of the first Italian artists to try oil painting, and he often painted religious frescos in churches and abbeys. One of the colors he used at that time was referred to as oltremare genuino, the "Color Beyond the Sea". Oltremare was very expensive because it had to be imported from Asia, where Buddhist temples were decorated with the color. Perugino once worked for a monk who had developed a technique for making this most expensive of all colors. The monk encouraged Perugrino to use it in paintings where blue was needed, such as on the robe of the virgin.
But the monk was very possessive of the valuable color, and he maintained a constant watch over Perugrino. So Perugrino decided to teach the monk a lesson. Working on the fresco, Perugrino had a basin of water for dipping his brush. He also had a container for the oltremare powder to be used in the painting. At every second stroke, Perugrino would dip his brush into the blue powder, and then dip it into the water basin, depositing blue pigment into the water. The monk filled the pigment container over and over again with oltremare, wondering why the painting was taking so much blue!
Much later, Perugino came to the monk with the basin of water in hand. Then he said to the monk, "It is better to trust those who are honest, for if they wish, they can cheat distrustful people such as yourself any time they decide to."
We now have a synthetic version of oltremare genuino. It is called Ultramarine Blue.