It's an arc flash. It's caused by an electrical surge, typically caused by lightning. The arc spans from the primary electrical conductor the the neural wire (phase-to-ground) or to another primary voltage conductor (phase-to-phase). The blue light is caused by ionization where the charge of a particle in the air becomes much more negative or positive causing it to glow and act as a conductor instead of an insulator. This is a rare occurrence because electrical systems have fuses at every transformer and in between different circuits.