The protagonist, atmosphere and outcomes are marked by darkness of soul, reflected in the coloration of the novel, as well as themes, actions. It is NOT nihilism, but may involve ethical nihilism. In some fashion, the occult is a main part of the story line. These books often explore the psychology behind the crime, and the mind of the criminal. A church or a religion is central to the main story line.
Romance is probably more important to the story than the mystery.
Based on today's technology, it may include forensics, a medical examiner, fingerprint experts, etc. Goes into detail on how science helped solve the mystery. The good guys are after a serial killer. Not generally for the squeamish. Ghoulies and ghosties, and long-legged beasties.