I work in this field, having low blood pressure is not a bad thing (obviously bad if its too low) but if its above 100/60 ish you will be fine. Probably the reason they turned you away was because giving blood makes you (well me anyway!) a bit light headed, thats why you already need to be hydrated and they make you have some sugar after. And low BP can cause this also. If you combine the two then they're at risk of having a passed out vegan on their floor!
What is your BP? if you dont mind me asking!
Thanks for the replies guys!
My BP was 110/68.. I don't really know what that means?