Definition of blue light:
(n) :
(countable) A flashing light, usually fitted to an emergency vehicle.
(v) :
(informal, intransitive) To travel quickly in a police or ambulance vehicle with the lightbar (and possibly the siren) activated.
(n) :
Used other than figuratively or idiomatically: see blue, light.
(n) :
(historical, chiefly nautical) A mixture of chemicals (including nitre, sulfur and antimony) used in the eighteenth and nineteenth centuries for night-time signaling and general illumination.
(n) :
(US, colloquial, now historical) A New England federalist, who opposed the Anglo-American War of 1812; (loosely), a federalist.