The blood pressure when blood comes out of body

Blood pressure is the pressure above barometric pressure. When you pump up a blood pressure cuff, or a tire for that matter, you don't have a gauge with a line at 760 torr / mmHg; you're measuring the pressure relative to the air in the room.

Note that blood pressure is only 16% of normal atmospheric pressure. Imagine what your blood pressure would be if you were the victim of explosive decompression in outer space!