ROUBAIX, France—A French brewer has started using algae with a naturally-occurring pigment to turn their beer blue. The beer, with the brand name “Line,” is the result of a tie-up between a firm that wants to popularize the algae as a dietary supplement, and a nearby craft brewery that was looking for a way to make its beverages more distinctive. The beer is selling well, said Sebastien Verbeke, an employee of Hoppy Urban Brew, which makes the drink. “It’s getting an enormous amount of interest and curiosity on the part of the public,” he said. The blue tint comes from spirulina, an algae that is grown in basins by a company called Etika Spirulina in northern France. The component of the spirulina that gives the blue colour, called phycocyanin, is then added to the beer during the brewing process. Tasting a freshly produced bottle of the beer, brewery employee Mathilde …