Preheat oven to 350°; position oven rack in middle of oven; spray the bottom only of a 13x9 inch metal baking pan with nonstick cooking spray.
Melt ½ cup butter in a small saucepan over low heat.
In a big bowl beat the cake mix, melted butter, and 1 egg using an electric mixer set on medium speed until well blended (will come together as a thick dough).
Pat the dough evenly into the prepared pan; set aside.
Melt the remaining butter in a saucepan set over low heat.
In a big bowl beat the cream cheese, lemon juice, and lemon zest with a electric mixer set on medium speed until blended and smooth.
Add in melted butter and remaining 3 eggs; blend until smooth.
Add in the powdered sugar; blend until smooth; stir in the blueberries.
Spread lemon-blueberry mixture over prepared crust.
Bake 42-45 minutes (do not overbake, bars should be slightly gooey).
Transfer pan to a wire rack; cool completely.
Cut into bars; cover and store in the refrigerator.