Jennifer Forness, a 39-year-old in Groton, Conn., felt her life falling apart earlier this year. Her husband announced he wanted a divorce. She lost her job as a store clerk. She developed health problems from the stress.

Then one night she discovered a website selling products for hoodoo—an ancient belief system based on spells, potions, balms and curses that slaves developed long ago in the Deep South. Ms. Forness ordered several items and instructions for performing certain rituals. She also had a telephone session with...