¼ cup (2 ounces) blanc vermouth (such as Dolin Blanc Vermouth de Chambéry)
0.33 ounce dried hibiscus flowers (about ⅓ cup)
1⅓ cups ice cubes (about 6 ¾ ounces)
1¼ tablespoons (0.67 ounce) London dry gin (such as Bombay Sapphire)
1¼ tablespoons simple syrup
¾ tablespoon (0.33 ounce) Campari
Combine vermouth and hibiscus flowers in a medium measuring cup, pressing to submerge flowers in liquid. Cover and let stand at room temperature until vermouth is bright red and tastes tart and floral, at least 24 hours or up to 48 hours.
Pour vermouth mixture through a fine wire-mesh strainer into a bowl, pressing flowers with the back of a spoon to extract about ½ cup plus 2 tablespoons (5 ounces) liquid. Reserve flowers for garnish.
Combine hibiscus-infused vermouth, ice cubes, gin, simple syrup, and Campari in a blender; process until smooth, about 30 seconds. Pour evenly into 6 coupe glasses. Garnish with reserved hibiscus flowers.
