Your blood is more valuable in your body than it is in someone else's body. Why harm your health to save someone you don't even know, especially if they're probably an enemy?
I'm not giving my blood to someone I don't know. There is no essential difference between blood and meat. Would you cut off your flesh and feed it to a starving stranger? Oh, I can't do that. Don't be so sympathetic.
That doesn't mean I'm a selfish person. If any worthy person needed blood (say, an SS), I would definitely donate it. But a strange enemy? No way.