Blue light (pyrotechnic signal)

Blue light is an archaic signal, the progenitor of modern pyrotechnic flares. Blue light consists of a loose, chemical composition burned in an open, hand-held hemispherical wooden cup, and so is more akin to the flashpan signals of the Admiral Nelson era than the modern, encased signal flares, often launched by mortar or rifle and suspended by parachute. Widely used during the eighteenth and nineteenth centuries for signaling by the world's military forces, and for general illumination in the civilian sector, blue light was remarkable for its use of poisonous arsenic compounds (realgar and orpiment), which contributed to its replacement by safer flares in the early twentieth century.

"Blue light" was a derisive nickname given to military officers of the 18th and 19th centuries, whose evangelical Christian zeal burned as brightly as its namesake signal, to the chagrin of those less ardent who were subject to the perceived ostentatious piety. Confederate Gen. Stonewall Jackson carried the nickname "Old Blue Lights" during the American Civil War because of his overt religiosity.

Blue light was famously mentioned in accounts of the H.L. Hunley, the Confederate submarine which became the first to sink an enemy vessel, the USS Housatonic, on February 17, 1864, during the Civil War. Such blue light has been repeatedly misidentified by authors and researchers of the Hunley story as a blue lantern, since they failed to realize the 1864 meaning of "blue light" as it was known to eyewitnesses who testified to its use during the battle between the Hunley and Housatonic. Pyrotechnic blue light was commonly used by the vessels of the Federal South Atlantic Blockading Squadron off of Charleston, South Carolina and would have been a familiar sight to both Union and Confederate soldiers and sailors.

...
Wikipedia