The last few times I donated blood, I've been trying to see how it affects my running. A few months ago, I tried biking about five hours after blood donation. That felt alright. Next time too, I biked the same distance (~4 miles) and about 40 hours later, went for a 7 mile run. That too went off without problems - no blood, no tears, only sweat (it was a hot day). This month, I went for a 3.5 mile run at about 8:00 minute/mile pace and after about 5 hours, went in for the blood donation, and about 3 hours after that, went for another 3.5 mile run. I consciously dropped my pace to around 8:40 minutes/mile from my usual easy run pace of 8:00 minutes/mile since this was a first experience for me. I was mentally prepared to slow down to 9+ minute pace and even walk if needed, but, to my pleasant surprise I had no problems whatsoever. Indeed, at the end of the run, I felt that I could have gone at 8:00. Next time I think I will increase either the distance or the speed, most likely the distance.
One of the things that happens with blood donation is that you instantaneously lose a pound of body weight. Maybe that makes you a bit faster. Of course, don't tell that to the nurse at the blood center. This time, when one of them found out that I run, she made it a point of telling me - "Absolutely no running for at least 24 hours."
Browsing the web, I came across a wide spectrum of experiences that other people had - from feeling absolutely normal to feeling very weak, from nausea to headaches. If you are racing, then blood donation seems to have more of an effect than if you are simply out for a jog. However, I wasn't able to find any specific study on this subject. I'll post links here if I find them.