Green’s Dictionary of Slang

(UK Und.) anyone who wears a long, blue overcoat (the uniform of a parish beadle).

1607		Middleton Michaelmas Term III v: He [...] keeps a drab in town, and, to be free from the interruption of blue beadles and other bawdy oficers, he most politely lodges her in a constable’s house.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue n.p.: Beadle. A blue roquelaure, to sport a Beadle, to fly a Beadle, &c. &c. Cant.