The symbolic three-eye protective design featured in the pendant is inspired by the evil eye. The evil eye is a superstitious curse or legend, believed to be cast by a malevolent glare, usually given to a person when one is unaware. It dates back at least to Greek classical antiquity, 6th century BCE where it appeared on Chalcidian drinking vessels, known as ‘eye cups’, as a type of apotropaic magic. Many cultures believe that receiving the evil eye will cause misfortune or injury, while others believe it to be a kind of supernatural force that casts or reflects a malevolent gaze back-upon those who wish harm upon others (especially innocents). Talismans or amulets created to protect against the evil eye are also frequently called “evil eyes”. Older iterations of the symbol were often made of ceramic or clay, however, following the production of glass beads in the Mediterranean region in approximately 1500 BCE, evil eye beads were popularised with the Phoenicians, Greeks, Romans and Ottomans.
Blue was likely used as it was relatively easy to create, however, modern evil eyes can be a range of colors. Read more: https://en.wikipedia.org/wiki/Evil_eye