If someone gets shot the blood droplets will hit the ground. With enough time the Water from the blood will evaporate changing the surface tension gradient across the the droplet. These gradients can cause convection inside the drop and pushes the red blood cells towards the outer part of the drop leaving different patterns. The patterns depend on what parts of the blood are deposited.
Looking at the patterns it can show a difference between different types of red blood cells e.g. anemic vs. normal blood.