1940s; earliest use found in James Lees-Milne (1908–1997), architectural historian and conservationist. From the colour commonly used for the Virgin Mary's cloak in devotional images.
Definition of Mary blue in US English:
Mary blue
noun
A light or medium blue colour.
Origin
1940s; earliest use found in James Lees-Milne (1908–1997), architectural historian and conservationist. From the colour commonly used for the Virgin Mary's cloak in devotional images.