Green’s Dictionary of Slang

blue pigeon n.

[late 18C–late 19C] (UK Und.)

1. a thief who specializes in stealing the lead from roofs [like an avian pigeon, the thief ‘perches’ on a church roof].

2. (also blue, pigeon) small off-cuts of lead or similar materials, taken from the job in hand and sold off as perks by plumbers.

3. (also bit of blue) a lead bullet.

In compounds

blue pigeon flyer (n.) (also blue pigeon flier)

[mid-19C] a stealer of lead from the roofs of buildings; such a thief poses as a journeyman glazier, plumber or other workman who gets to the roof, strips off the lead and hides it (often by wrapping it round the body) before leaving the house.

In phrases

fly the blue pigeon (v.) (also fly the pigeon)

[late 18C–1900s] to steal the lead from a church roof; thus (blue) pigeon-flying, conducting such thefts.