Blackberry-and-Sage Shrub

In a large nonreactive pot (such as stainless steel), combine berries, sugar, and sage. Let stand 1 hour, stirring a few times. Cook over medium heat until warm to the touch, about 5 minutes. Add vinegars and continue cooking, stirring occasionally, until mixture comes to a bare simmer (do not let boil). Remove from heat; let stand at room temperature, uncovered, 24 hours. Remove and discard sage. Strain shrub base; reserve pickled berries (which can be stored in an airtight container in refrigerator up to 1 week) for another use. Transfer shrub base to another airtight container; refrigerate until cold, at least 1 hour and up to 1 month.

Fill glasses halfway with ice. Top with 1 part shrub base and 4 to 5 parts sparkling water. Stir once; garnish with more sage and fresh berries, if desired. Serve immediately.