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.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Blue Devils * Blue PigeonsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes