Blackout is a temporary loss of consciousness, most often caused by an inadequate supply of blood to the brain (fainting), but also may occur during an epileptic seizure or during a mild stroke.
If a victim has not regained consciousness within a few minutes, and has not completely recovered within 15 minutes, summon medical help and treat for unconsciousness.
If a blackout lasts longer than several minutes, it may be caused by an underlying illness, and the victim should seek medical advice on recovery.
Repeated blackouts are also an indication that further medical attention is needed.

An alcoholic blackout is a little different.
Alcoholics will often suffer memory loss (amnesia) for a period during which they have been drinking heavily. Although they may not have lost consciousness at any time, this for-gotten period is referred to as a blackout.
A history of alcohol-induced blackouts is strongly suggestive that a person suffers from alcoholism.