When i stand up my vision goes black-ish and i feel dizzy. what causes this?

Low blood pressure: When you sit or crouch, the blood is squeezed out of the lower part of your body. Then when you stand up it pools in these parts and lowers the pressure in the upper part, especially the brain. This can cause visual blur, dizziness and a feeling of faintness. This might also mean you are anemic. If this happens most of the time, check with your family doctor for diagnosis.