In a medium sauce pan, combine black cherries, cranberries, sugar and 1/2 cup black cherry cranberry juice. Bring to a boil and cook for 5-10 minutes, until berries are popping and mixture is beginning to thicken. Remove from heat.
In a microwave-safe measuring cup, combine gelatin and 1/2 cup cranberry juice. Heat in the microwave for about 30 seconds, until gelatin is dissolved. Add gelatin mixture and remaining 1/2 cup black cherry cranberry juice to cherry cranberry mixture and stir well. Pour into prepared pan.
Refrigerate overnight, or until gelatin is firm.
Slide a spatula around the edge of the pan to loosen jelly and turn out onto a serving platter.