Beat egg in a medium-sized mixing bowl until light lemon-colored and fluffy.
Beat in blueberry preserves.
Add cake mix and water alternately in small amounts, beating well after each addition.
Beat a total of 5 to 6 minutes.
Pour into a wax paper lined or greased and floured 8 in. (20 cm) cake pan, and bake at 350°F (175°C) for 20 to 25 minutes.
Cool cake in pan for 10 minutes.
Turn cake out onto rack and cool completely.