BLUE (common in different forms to most European languages), the name of a colour, used in many colloquial phrases. From the fact of various parties, political and other, having adopted the colour blue as their badge, various classes of people have come to be known as "blue" or "blues"; thus "true blue" meant originally a staunch Presbyterian, the Covenanters having adopted blue as their colour as opposed to red, the royal colour; similarly, in the navy, there was in the 18th century a "Blue Squadron," Nelson being at one time "Rear-Admiral of the Blue"; again, in 1690, the Royal Horse Guards were called the "Blues" from their blue uniforms, or, from their leader, the earl of Oxford, the "Oxford Blues"; also, from the blue ribbon worn by the knights of the Garter comes the use of the phrase as the highest mark of distinction that can be worn, especially applied on the turf to the winning of the Derby. The "blue Peter" is a rectangular blue flag, with a white square in the centre, hoisted at the top of the foremast as a signal that a vessel is about to leave port. At Oxford and Cambridge a man who represents his university in certain athletic sports is called a "blue" from the "colours" he is then entitled to wear, dark blue for Oxford and light blue for Cambridge.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)