1. Preheat oven to 350°F (176°C) and prepare a 9×13-inch cake pan with non-stick baking spray.
2. Add the dry cake mix and lemon zest to a large mixer bowl and whisk to combine.
3. Add the additional ingredients listed on the box and follow to package instructions for mixing and bake time.
4. While the cake is baking, add the blueberries, sugar and water to a food processor and puree until smooth.
5. Pour the puree through a fine mesh sieve to remove the blueberry skin bits.*
6. Combine the puree with the lemon juice and sweetened condensed milk.
7. When the cake comes out of the oven, poke holes all over the cake with a straw, the end of a wooden spoon, or something similar.
8. Pour the puree mixture over the top of the cake and let it soak into the cake and holes.
9. Refrigerate the cake while it cools and soaks up the moisture.
10. To make the frosting, combine the butter and shortening with a mixer until smooth.
11. Add about half of the powdered sugar and mix until smooth and well combined.
12. Add the vanilla extract and lemon juice and mix until combined.
13. Add the remaining powdered sugar and mix until smooth.
14. Spread the frosting evenly over the cooled cake, then top with lemon slices, zest and blueberries, if desired.
15. Refrigerate cake until ready to serve. Cake is best well covered for 2-3 days.