The social media posts started in May: photos and videos of smiling people, mostly women, drinking Mason jars of black liquid, slathering black paste on their faces and feet, or dipping babies and dogs in tubs of the black water. They tagged the posts #BOO and linked to a website that sold a product called Black Oxygen Organics. Black Oxygen Organics, or “BOO” for short, is difficult to classify. It was marketed as fulvic acid, a compound derived from decayed plants, that was dug up from an Ontario peat bog. The website of the Canadian company that sold it billed...