Sitting on bench seats in a retrofitted old hearse, stuck in quintessentially choked Los Angeles highway traffic, I listened to a woman narrate the macabre details of an 80-year-old murder. The guide, Blaze Lovejoy—whose business card identified her as a “dark storyteller” and “Manson specialist”—had already driven us through Hollywood. Now she was bringing us to downtown Los Angeles, the heart of this particular darkness. Lovejoy wore combat boots with flames across the toes and had a microphone headset on, her British accent giving the narration a Vincent Price vibe: “A Hollywood dream turned to a real-life nightmare, with a...