To black out something is to delete or obscure it. The verb can also be used to put something in complete darkness, either by blocking the light or disabling electricity. A person can black out if he or she loses consciousness or vision for a period of time. The verb also means to institute a blackout on something.
A blackout can be a precaution against an enemy, where the lights are kept off to keep a people’s presence hidden. It can also be the period of time where electricity has failed in a city, and the period of time a person can lose consciousness. Alternatively, a blackout can be when an activity, usually a sporting event, is banned from being televised.
In short, black out is a verb and blackout is a noun. Use each spelling accordingly in your writing. Notice that a hyphen is never to be used. The single word spelling is appropriate when using the term as an adjective.
Examples
According to Public Works Director Keith Couture, the city does black out and erase obscene graffiti in public areas, but only if a resident complains about it. [CBC]
Speculation has swirled over the past month as to whether the 33-year-old blacked out or had some kind of medical episode before he lost control, rather than his explanation, that the steering jammed. [The New Indian Express]
Early Sunday morning, the two sides traded barbs about the reasons for the blackout, and Fox News urged its fans to switch to another television provider. [CNN Money]
Customers in Beverly Hills, Coldwater Canyon, Century City and West Hollywood were affected by the massive blackouts. [The Hollywood Reporter]