I didn't have pre-existing hypertension (my BP is always on the high end of normal), but when I was in my 20s, the combined pill used to send my BP through the roof. I was told I would have to be closely monitored in pregnancy as it was the oestrogen in the pill causing my BP to rise. I was on the mini pill for years, and then successfully swapped to a low oestrogen combined pill. As it turned out, my BP was absolutely fine in pregnancy and since I had my DDs, the combined pill no longer messes with my BP and I can take the full strength again.
My Mum has had hypertension since the age of 24 and was still able to take the pill as a method of controlling her period. She was on beta-blockers for the hypertension at the same time.