It makes sense that giving blood would affect your performance. Your muscles need oxygen to function, and they get this from the red blood cells. Donate a pint of blood, and you reduce the amount of red cells by 1/8th. Therfore less oxygen will be available for the 12 weeks or so it takes the body to renew them.
However, how much effect it will have on you will depend a lot on your level of fitness/training. Heavy training depletes the store of red cells anyway, and someone doing a lot of races will probably be a bit low anyway. To balance that, a well trained body is very efficient at utilising oxygen - more so than mere mortals. If you are a slow runner, you probably don't use all the available oxygen anyway, so would notice the difference less.
It will be a combination of how hard you have been training, how well your system can transfer the oxygen, your general health and how well you rehydrate after the donation that will dictate the final effect.
In short - I personally would not donate in the 2 weeks prior to a race, as I would probably feel a bit lethargic, but would find a session elsewhere a coupleo of weeks after it, once I had fully recovered.