The Clue of the Black Flower by Carolyn Keene

"You will sell me the flower!" insists the strange man. Louise Dana is baffled as to why the man would value an artificial black orchid. Shortly after Jean and Louise arrive at summer camp, the orchid is stolen. The girls learn that not only does the orchid have a hidden value, but may provide a clue to a pair of twins who went missing years before in South America. What is the amazing secret of the black orchid?