Blood is released onto the battlefield whenever a troop is hurt by bullets, explosions, CQC or barbed wire. So, as you've learned, blood does not come out of thin air. If you come across some suspicious tracks, you have yourself a camouflaged unit sneaking around! Blood splatters come in different patterns, so collect 'em all!
Pay attention to these, as they can show where enemy infiltrators or snipers are. Using Concentrate Fire on the area near the blood splatter can detect invisible units, and if you click on one, it will cause your troops to target the invisible unit.
Blood splatters are usually accompanied by screaming units, enemy or friendly. Blood can also come out of vehicles when a passenger is shot.