Blood

For nothing, as far as today's recipes go. The in-game text still says blood is used to make medkits, but neither its own data nor any recipe on this site lists it as an ingredient, and no blueprint makes it. The line most likely survived from an older crafting system. Today's Large Medkit is built from two medical syringes and low grade fuel, with no blood anywhere in it.

So if it ever turns up in a box on a modded server or in an admin's spawn list, treat it as a curiosity. There is no recipe to save it for, and it is not worth a slot in your stash. Whatever actually heals you is on the Medical page, from the cheap bandage up to the medkit.