After the weapon is fired, air races into the barrel once the bullet has left the muzzle. This vacuum can pull in trace amounts of materials from the environment.
Police can use blood and tissue which have entered a gun barrel through blowback in an investigation.
Blowback can also refer to the combination of gasses, dirt, and debris (unburnt powder, metal shavings) that most firearms produce upon firing. This can cause great irritation to the eyes and most ranges or organizations suggest or require the use of safety glasses when firing pistols.