I'd been advised by a doctor not to donate many years ago when my blood pressure was quite low. That said I've never had problems when getting a standard blood test done but they take a much smaller quantity I guess.
My body doesn't react well even to small samples, and as such I've never given blood. My BP is normally slightly high but hr low. I remember having small samples taken the day after Skyline in 2015 and passing out on the nurse - who just so happened to Tom Westgate's wife. So I at least got to hear about the back of the field being hypothermic and being MR'd off, which I'd missed on the day. Fine with needles, the reaction is elsewhere.
Same, I'm fine with the needles bit too. I had a similar experience with a blood sample a few years back, so maybe just sensitive to it in some other way. Oh well, the nurse made it clear that it's not really worth another try, so won't be donating again any time soon anyway.