1) English ghost 2) Northumbrian folkloric being 3) Supernatural legend 4) Titmouse 
A bluecap is a mythical fairy or ghost in English folklore that inhabits mines and appears as a small blue flame. If miners treat them with respect, the bluecaps lead them to rich deposits of minerals.{sfnp|Allen|2005|p=24|ps=} Like knockers or kobolds, bluecaps can also forewarn miners of cave-ins. They are mostly associated with the Anglo-Scotti...
• (n.) The bluepoll. • (n.) The blue bonnet or blue titmouse. • (n.) A Scot; a Scotchman; -- so named from wearing a blue bonnet.
species of fairy wren (q.v.).[1 related articles]
Blue'cap` noun 1. (Zoology) (a) The bluepoll. (b) The blue bonnet or blue titmouse. 2. A Scot; a Scotchman; -- so named from wearing a blue bonnet. [ Poetic] Shak. 
Bluecap was British Second World War slang for an anti-sabotage security guard.