(The), applied to troops.
The Oxford Blues.
The Royal Horse Guards were so called in 1690, from the Earl of
Oxford their commander and the blue facings. Wellington, in one of his
despatches, writes:- “I have been appointed colonel of the Blues.”
“It was also known as the `Blue Guards' during the campaign in
Flanders (1742-1745).” —Trimen: Regiments of the British Army.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894