New York Sky Turns Bright Blue After Explosion at a Con Edison plant.

A giant plume of smoke filled the New York City sky around 9:12 p.m., and turned it blue.
The bizarre illumination was sparked by an “electric arc flash” tied to faulty equipment at a Con Edison substation

When an electrical current moves through air in an electrical arc, it alters the molecules in the air, charging them with energy, splitting them into atoms and ionizing them. Electrons, excited by the extra energy, get separated from their atoms. The result is a plasma, or charged gas.

When the electrons and ions in a plasma recombine, the atoms glow and emit visible light. The same process is behind fluorescent light bulbs, neon signs and lightning. In all those cases, the color of the light is characteristic of the elements and atoms involved.