I am hypertensive and on multiple meds for it. I returned to cycling last April.
I learned pretty early on, after strenuous rides, that I needed to do adequate cooldown laps before dismounting or I would start to black out. I was fine as long as I kept riding, it was only when I stopped that I had the problem. Having my suspicions, I checked my BP immediately after a hard ride and found it to be about 70/40. Explains a lot to me. What I figured out was that a large amount of vasodialation occurs under strenuous exercise leading to a drop in my BP.