Read up on POTS and see if that sounds similar to what you are experiencing. POTS isn't all that common but it is a form of tachy related to posture/position.
blacking out because you're getting up too quickly is usually a sign of hypotension, not hypertension.
it might not be either. make sure you're drinking enough water. it simply could be a case of dehydration.
If you get up fast, it shouldn't get to the point where you black out. It happens to all of us sometimes but the cardiovascular system should compensate quick enough to get oxygen levels raised into the brain. What is your blood pressure like? and are you healthy and fit?