In the late Middle Ages, rumors began to spread throughout Europe regarding blood miracles associated with the relics of martyrs. Centuries-old blood, pulverized or solidified and black in color, was said to return to its original bright red color, or else to liquefy or bubble under certain circumstances or on certain dates in the liturgical calendar. With the Reformation, in Protestant countries most of these relics were either destroyed or forgotten. In Catholic countries, on the contrary, blood miracles multiplied, reaching a peak between the seventeenth and eighteenth centuries. This article reconstructs the debate that sprang up in eighteenth-century Europe over the blood of Saint Januarius and the attempts made to disprove its miraculous properties, often not in written works, but by staging highly theatrical demonstrations. It examines the way in which, with phenomena as complex as miracles, the activities of testing alleged facts, creating elucidative models, and staging imitations intertwined over the centuries, often overlapping and becoming confused.