Preheat your oven to 350°F (175°C). Grease your 9x9-inch baking pan or line it with parchment paper, leaving some overhang on the sides for easy removal.
In a large mixing bowl, beat together the softened unsalted butter, granulated sugar, and brown sugar using a hand or stand mixer on medium speed until light and fluffy, about 3-4 minutes.
Beat in the eggs one at a time, making sure each is fully incorporated before adding the next. Then add the vanilla extract and freshly grated lemon zest.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing on low speed just until combined. Avoid overmixing.
Gently fold in the blueberries using a rubber spatula, being careful not to crush them.
In a medium bowl, beat the softened cream cheese until smooth. Add the powdered sugar and lemon juice, then mix until creamy and well combined.
Spoon half of the blondie batter into the prepared pan and spread evenly. Drop spoonfuls of the cheesecake mixture over the batter, then dollop the remaining blondie batter on top. Gently swirl the cheesecake through the blondie batter to create a marbled effect.
Bake for 35-40 minutes or until the blondies are set and a toothpick inserted near the center comes out with just a few moist crumbs. Let cool completely in the pan on a wire rack before slicing.