The Thin Blue Line is a 1988 American documentary film directed by Errol Morris. It examines the events leading up to the 1976 murder of a Dallas police officer and the subsequent conviction of Randall Dale Adams. The film investigates the reliability of the testimony of a key witness and the possible innocence of Adams
From Wikipedia
The "thin blue line" is a term that typically refers to the concept of the police as the line between law-and-order and chaos in society. Blue refers to the color of many police uniforms.