1. Prepare grape-cherry syrup. In a medium non reactive saucepan, over medium-high heat, bring grape juice and black cherry juice to a simmer. Decrease heat to medium and continue to cook for about 1 hour, stirring occasionally, until juice reduces down to 1 cup (check yield by carefully pouring hot juice into a glass measuring cup). Cool syrup for 1/2 hour, then refrigerate in a covered jar until you make soda. (Syrup will keep, refrigerated, for a week.)
2. To make soda: Fill a 12-oz. glass halfway with ice cubes. Add 1/4 cup chilled grape-cherry syrup to glass and top with 1 cup sparkling water or club soda. Add a straw, stir, and serve immediately.
Repeat to make up to 4 sodas.
Source: Hannaford fresh Magazine, July - August 2007