What is the X in X-mas?
also black-out, 1908 in the theatrical sense of a darkened stage, from black + out. Figurative sense of "loss of memory" is 1934 (verb and noun); as a dousing of lights as an air raid precaution, it is recorded from 1935. Verbal phrase black out, in reference to printed or written matter deemed objectionable and covered in black ink, is attested from 1888.
blackout black·out (blāk'out')
Temporary loss of consciousness due to decreased blood flow to the brain.
Temporary loss of memory.
The complete loss of electrical power in a particular area. Blackouts can result from a natural disaster, a manmade catastrophe, or simply from an excess of energy demand over supply. (Compare brownout.)
Note: Rolling blackouts to match supply and demand have become increasingly common in the United States.
A period during which discount or favorable prices on airlines are arbitrarily canceled: flight from LA to NYC for free (depending on availability, blackout dates and routings) (1990s+)