Blacking out or fainting is caused by a temporary lack of enough blood to the brain. If you are aware that a fainting spell is coming, sit with your head between your knees for a moment or two to increase the flow of oxygen to the brain.
Blackouts (especially when combined with other symptoms) can signal serious conditions such as heart failure, high blood pressure, or heart rhythm problems. Fainting can also be a sign of less serious conditions, such as fatigue, anemia, hunger, or prolonged standing. If you are experiencing blackouts, call your doctor for evaluation.