I train in the early morning before work so I do what rmtt stated above. I'll typically donate around lunch break, then drink plenty of fluids and eat well the rest of the day and evening and then train the next morning same as usual. I haven't noticed any decrease in strength or endurance levels utilizing this system.
Gave blood for the first time in years last week. Didn't work out until next day.
Strength seemed a bit weaker but not bad. Where I felt a significant change was cardio. Did a bit of research and since my red bloodcells were high, when I gave blood it made it go down (good thing) but that lowering also doesn't allow as much oxygen into the body as you are used to. That's why athletes train at high altitudes to increase their red blood cells.
Just what I read and seemed to explain my situation anyway.