The idea here is to give a distinct but light flavor to water--so it's truly refreshing, rather than sweet and intense like soda.
Put blackberries and sage in a large cocktail shaker. Add 1 tbsp. water; mash well with a muddler or end of a wooden spoon.
Stir in sparkling water, cover, then strain into an ice-filled glass (for a clearer drink, pour through cheesecloth).