Forty-nine times the Bible mentions a perfect, pure blue, a color so magnificent and transcendent that it was all but impossible to describe. Yet, for most of the last 2,000 years, nobody has known exactly what “biblical blue” — called tekhelet in Hebrew — actually looked like or how it could be re-created. At the time of the Second Temple, which towered above Jerusalem until it was destroyed by the Romans, a blue dye of the same name was used to color the fabric used in the clothing of the high priests. Jewish men are still commanded to use a...