In general during peacetime, a blackout is an unintentional total electrical power outage. It is sometimes contrasted with the term brownout which is a reduction in power, and selected power outages, when demand exceeds supply.
However a blackout can be intentional, for instance collectively minimizing external light (during war), especially upwards directed (or reflected) light, to "hide" from enemy aircraft (in the case of an occupied country: the enemy of the occupying force).
See also
Alternate meanings