To black out is to lose consciousness, or to cut all the lights. Black out at the sight of blood.
The wartime sense of a whole city going dark gave the noun that everybody still knows. To black out from drink means losing the memory rather than the consciousness, which is a different thing.
1phraseblack outTo lose consciousness, or to cut all the lights.
“He would black out if he stood up too fast.”
black out is a phrase.
Practised in Phrasal Verbs