A "blue" or "true blue" used to mean a Tory, or member of the Conservative Party in Britain. A "blue" is also a college athlete, chosen to represent his university (shortened to "varsity") in sports. What do these blues have in common? Unswerving loyalty to party or school, etymologists say.
It means "a member of any noble or socially prominent family." The word has its aristocratic origins in Spain. According to Web Garrison's "Why You Say It," a few patrician families living in the mountains of Castile kept the fair complexions of the original Spaniards. These aristocrats avoided intermarriage with the swarthy Moorish invaders. They also stayed out of direct sunlight. Their pale white skin had a bluish cast from the blood vessels close to the skin in their faces and hands. This trait was so marked that people began to speak of Castilians as "blue bloods."
The term, in translation, migrated to other European countries whose nobles wished to distinguish themselves from commoners.
Kentucky may have the Atlantic coast (500 miles away) to thank for this term. The grass is really only tinged blue, but experts say early settlers on the East Coast had found a grass with distinctly blue leaves. When the settlers moved west to Kentucky, they found a grass similar to their "bluegrass" and gave it the same name. Bluegrass also refers to the country music - played on lawns, perhaps?
In early England, it was customary to bind Parliamentary documents in a distinctive blue plush. So distinctive were these volumes, in fact, that any authoritative text - no matter what the color of its binding - was called a "blue book." This came to include registers of the socially prominent.