Blood is a Passive Skill in Vampyr. It's also essential for the survival of the Vampire, who feeds on the blood of the living.
When Jonathan E. Reid checks the citizen's blood, each one of them has a certain blood quality. Blood quality suffers from diseases. Jonathan also can take samples and find a treatment, should a citizen be ill. Blood quality also is important, should Jonathan decide to feed on a citizen. Feeding on high quality blood will result in higher XP gain. Healing people will not only give their blood quality benefits, but also will keep the district's Health Status up. Once a district turns to hostile (worst case!), the point of no return has been reached and the area will be swarming with dangerous encounters from now on throughout the rest of the game.