Blue Devils

The 1811 Dictionary of the Vulgar Tongue contains a wealth of information, including, the following definitions/explanations:

BLUE FLAG. He has hoisted the blue flag; he has commenced   publican, or taken a public house, an allusion to the blue aprons worn by publicans. See ADMIRAL OF THE BLUE.

BLUE DEVILS. Low spirits.


For more definitions from this source, see Bears and Bulls