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.)