Blue is a common color to represent femininity. Thus a girl or woman wearing blue is a sign that she is fair and gentle. This could have started at least as far back as ancient Greece and Rome, and why the Virgin Mary is often depicted in a blue robe. The logic was that blue symbolizes peace, serenity, kindness, and other such aspects that were considered womanly virtues. (In the Mediterranean, it was also a sign of high rank, as the dye/paint color had to come from indigo, woad, or lapis lazuli which must be imported from a great distance and was therefore quite expensive.)