A blackout is the loss of the electricity supplied to a certain area.
During the Federation-Dominion Cold War, Admiral Leyton ordered Red Squad to sabotage the global power grid on Earth, causing the whole planet to suffer a blackout. (DS9: "Homefront", "Paradise Lost")
Throughout the Battle of Cardassia, the Cardassian Rebellion sabotaged Cardassia Prime's communications, transportation and power systems, causing a planet-wide blackout. This disabled the Female Changeling's ability to command the Dominion ships during the battle, ultimatly resulting in the allies' victory. (DS9: "What You Leave Behind")