1. In a large bowl, combine lemon gelatin with 2 cups boiling water, stirring constantly until completely dissolved, about 2 minutes.
2. Stir in 1 1/2 cups of the blueberries and the yogurt until smooth.
3. Cut cake into 14 (1/2-inch) slices. Cut each slice into 3"1-inch rectangles.
4. Arrange rectangles upright around edge of an 8-inch springform pan; arrange remaining pieces to cover bottom of pan.
5. Spoon in blueberry-yogurt filling.
6. Scatter remaining 1/2 cup blueberries over top. Cover; chill until firm, about 2 hours.