Lightly spray 9x5-inch loaf pan with cooking spray, then blot with paper towel; set aside. In large bowl, sprinkle gelatin on 1/2 cup cold grape juice to soften; let stand 1 minute.
Meanwhile, in blender, place blueberries and yogurt. Cover; blend on high speed until smooth. Add hot grape juice to gelatin mixture, and stir about 2 minutes or until gelatin is dissolved. Stir in yogurt mixture. Pour into loaf pan.
Cover and refrigerate until firm, about 4 hours. Cut into cubes and serve, or cover and refrigerate up to 2 days before serving.