A lot will depend on your readings when you were in. If they give you an exam where you are laying down and take your blood pressure 3 times, 1/2 hour apart and it indicates hypertension and they are vary close to your medical service bp, they "should" (and I would fight them on it) assume that more likely than not that it existed prior to departure from service. As they cannot dispell it, the regulations state that the benefit of the doubt will be awarded to the veteran. However, they will only monetarily retro your claim to the date you submit it, so I wouldn't waste any time. I would make a point of getting to the VA or using a local doctor using the VA guidelines.
The pressure shall be taken 3 times, 1/2 hour apart, alternating arms, for 3 consecutive days. As it is an actual physician, the raters can not dispute the findings.