There can be many different reasons why you blackout when you stand up. It could be due to loss of blood flow to the brain by standing up to fast, lack of nutrients, dehydration, injury, or other.
1 Additional Answer
Blacking out when standing up could be indicative of orthostatic hypotension. This means that the brain is temporarily starving for blood when standing, as the blood pressure is not being adequately maintained. A physician should be consulted to further diagnose.