Alright then realism...
First, it's peculiar that you ignore the point BeefBacon brought up. What about playing different characters in various hives? But besides that, no adult in a situation like the one DayZ represents would need to write down his/her bloodtype to remember it as it might be crucial for their survival. So the need of writing it down on paper is unrealistic. Many people nowadays even know the blood type of themselves and their children without carrying paper around just because it is an important information and therefore remember-able. Blood type of a game character... not so much. Wring it on a paper on the desk while playing, why wasting paper for something that could be easily been done in the game.
Now to realism. Where is the realism in any of the following:
1. A piece of paper takes as much in inventory space as a can of beans?
2. Last time I checked vegetables don't grow from seed to harvest within 20 minutes.
3. You place some snare traps on an arbitrary field and some minutes later you caught a rabbit? Good luck trying that IRL.
4. You eat a can of beans and you are hungry again an hour later and starve to death a couple hours later? I don't think so.
5. You log off, don't play the game for a week and your character is still alive? I might be wrong but a human who doesn't eat or drink for a week might have a difficult time to survive.
6. You break your legs, put on an improvised splint and a few hours later you are healthy as if nothing happened?
7. You suffer a great blood loss and after you stay hydrated and energized for one or two hours your blood is completely generated? yeah, not so much
this list could go on for quite some time. The point is, 100% realism would be boring and in game information are no in any form vital for survival IRL so what's the point in forcing a player to remember an character information any person in that situation would remember anyhow. DayZ is not a game of memory as far as I see it. A game is always a compromise between realism and condensed timing for certain processes.
So I totally agree with:
It would be more realistic if there is a chance you fail administering a blood bag to a person or even kill the person due to mishandling the infusion as this would not be that unlikely if someone who never did that before is trying to give someone an IV infusion rather than hiding information any person IRL would remember anyways.