Blackouts happen because alcohol when drank in large amounts and too quickly can impair the brain’s ability to make memories. That is why people claim to not recall what they did when they were very drunk. A blackout is different from passing out. Passing out happens when a person drinks until he is unconscious. This is because alcohol causes dehydration, blood vessels dilation leading to decreased blood pressure which then causes one to faint. If one drinks too much alcohol they can get alcohol poisoning and die!