For thousands of years in the Middle East, turquoise blue has been supposed to have the power to ward off the evil eye - the glance or stare of a malicious envier, which could sicken one in body or spirit. Apparently there are references to the evil eye in Sumerian clay tablets. A blue amulet is thought to ward this off by deflecting the glance back to the envier. (Note the use of turquoise-colored beads on donkey's today.)
One possibility as to why particularly blue was thought to have this property, is that the earliest pottery glazes developed by the Egyptians were blue. A new glaze would reflect light and so have a reflective property. Why were the earliest glazes blue? It seems that the earliest glass was made as an unintentional byproduct of copper smelting. When melted copper dripped off and mixed with sand (quartz pebbles) it produced glass with a blue-green color. Around 2000 B.C. the Egyptians made faience pieces - earthenware covered with a glaze made from a heated mixture of sand, salt, limestone and copper. This glaze was the first synthetic pigment. Faience was used decoratively as an imitation of the much costlier lapis lazuli in ancient Egypt.