Because the color Blue is associated with the Jewish faith and the Blessed Virgin Mary.
Mary and the Importance of the Color Blue
One of the most common colors used in statuary and iconography for the Blessed Virgin Mary is blue. With a quick image search online, nearly every image that pops up has Our Lady wearing something blue.
It’s so common that many Catholics take it for granted. The color blue has enough general associations that we can apply to Mary: it is the color of the sky/the heavens, and represents peace and tranquility. Tranquility befits Our Mother, who kept all the events of her Son’s life in her heart (cf. Luke 2:19, 51). Use of the color blue for the Virgin Mary goes beyond those basic associations. It has its roots in the Old Testament, Church history, and her apparitions throughout the last several centuries.
Blue in the Old Testament
The color blue had brief but significant mention in the Old Testament. Priestly garments were to include some blue, as well as altar cloths that covered the Ark of the Covenant and other sacred vessels in the tabernacle (Num 4:6-12). We could look at the blue cloths that cover the sacred vessels as a representation of her Immaculate Conception. Mary was full of heavenly grace from the instant of her conception. Only the purest articles were used in the tabernacle, the dwelling of God on earth. God chose the purest woman, then, when “the Word became flesh, and dwelt among us” (John 1:14). He specifically dwelt within one person, Our Blessed Mother, the New Ark of the Covenant! And, as the blue cloths protected the sacred vessels for thirty years, the ultimate Sacred One was enclosed by Mary; covered from the world, in a sense. Only at the appointed time (John 2:4) did He reveal Himself to the world.
The key point of the color’s significance is eleven chapters later:
“The Lord said to Moses: ‘Speak to the Israelites, and tell them to make fringes on the corners of their garments throughout their generations and to put a blue cord on the fringe at each corner. You have the fringe so that, when you see it, you will remember all the commandments of the Lord and do them, and not follow the lust of your own heart and your own eyes. So you shall remember and do all my commandments, and you shall be holy to your God.’” (Num 15:37-40)
The fringes (or tassels) described in Numbers 15 fits Our Lady perfectly! The tassel served to remind the Israelites of their priorities. There is no greater example of following the will & commandments of God than Our Blessed Mother. At the word of an angel, she offered herself and her fiat to God. Taking the Lord’s will as our own is a tremendously difficult task for those of us not immaculately conceived. Yet it is still within our reach, with the help of grace—and Mary’s fiat guides us in the way of grace.