He advises endurance athletes to think of the blood donation day as a rest day, and to tread cautiously the next day because hydration stores may not be replenished and delayed vasovagal effects may occur. Christie says though the performance decrement would be slight in an endurance athlete, blood donation should have virtually no effect on strength or short-burst activities."
You can read the whole article:
Donating Blood
What Active People Need to Know
http://www.physsportsmed.com/issues/2001/06_01/news.htm