I don't know anything about onions and blood pressure, but I do know foods rich in Vitamin K (usually green leafy veggies but also onions and green tea which of course is made from leaves) affect blood clotting. People who take warfarin are supposed to eat the same amount of those at the same times daily. (Right!) The more they eat, the less warfarin they need. I don't know if those foods could also affect BP.