1. Sprinkle gelatin and sugar over cold water in medium saucepan; let stand for 5 minutes to soften. Place over very low heat just until gelatin is dissolved.
2. Stir in orange juice and lemon juice. Chill until semi-set, about 30 minutes. Stir in melon balls and blueberries.
3. Pour into 4-cup mold; chill until completely set, about 1 hour. Dip mold into hot water and invert onto serving platter; shake to release mold.