By definition, blood pressure is the pressure at the exit of the heart (aorta). It is close to the pressure that you measure in your arm or wrist, provided that you keep it at heart level when measuring BP. The blood pressure is different at different locations in the body, e.g. higher in the feet and lower in the head when standing up.
In inversions, there can be a surge of blood to the head and a corresponding increase in local blood pressure. This short increase in blood pressure in the head (not at the heart) is the one causing potential risks of stroke. Once you are in headstand, the body will attempt to lower blood pressure. For uncontrolled hypertensives this adjustment can be impaired, putting them at greater risk than the rest of the population.
I don't know how the blood pressure measurements were done in the medical studies. Some of the earlier studies may have just calculated the hydrostatic pressure from the distance the head was below the heart. It would be interesting to get hold of the more recent studies, like Gilmore (2002) so see what they have done. Perhaps the doctors on this forum will comment.