Why is the Blessed Virgin Mary's color blue?

Answer:

"The older, classic and more representative color is dark blue," according to the Rev. Johann Roten, S.M, director of the Marian Library-International Marian Research Institute at the University of Dayton. "Mary's dark blue mantle, from about 500 A.D., is of Byzantine origin and is the color of an empress." Mary has traditionally been called "Queen of Heaven," as the Mother of God who was assumed into heaven upon her death.

On a more practical note, the color blue used in medieval painting was derived from lapis lazuli, a stone imported from Afghanistan of greater value than gold. Beyond a painter's retainer, patrons were expected to purchase any gold or lapis lazuri to be used in the painting. Hence, it was an expression of devotion and glorification to swath the Virgin in wide flowing gowns of blue (as well as a not-too-subtle expression of the patron's wealth).

However, it should be noted that Mary does not have an official color, and red has also been widely employed in her representation, particularly amongst German painters. Further, light blue is very popular, and is often (though perhaps as reflection, rather than motivation) associated with the color of the pure sky.