A lot of people think they aren't allowed to give blood (and a lot of them are right - anyone pierced, been to a malarial area, been to other exotic places, had gay sex or a partner who has, on various meds - all banned for various lengths of time).
Could be useful to say that the restrictions aren't as restrictive as you might think (eg being on the pill is usually fine) and may only ban you for a year so please look at the website and see if you are eligible after all?
I once worked somewhere where the head manager was known to give good appraisals to anyone who gave blood. His heart was in the right place but it did lead to people being made to defend why they weren't (and my boss really didn't want to tell the manager about his sex life...)