1. Preheat your oven to 350°F and grease a 9×13 inch baking pan.
2. In a large bowl, combine the flour, sugar, baking powder, baking soda, and salt.
3. Add the melted butter, buttermilk, eggs, and vanilla extract. Mix until just combined.
4. Fold in the blueberries gently, taking care not to break them.
5. Pour half of the batter into the prepared pan and spread it evenly.
6. In a separate bowl, beat the cream cheese, powdered sugar, and lemon zest until smooth.
7. Drop spoonfuls of the cream cheese mixture over the batter, then spread gently with a spatula.
8. Pour the remaining batter over the cream cheese layer and spread evenly.
9. For the crumb topping, combine 1/2 cup flour, 1/2 cup sugar, and 1/4 cup cold butter in a bowl. Mix with your fingers until the mixture resembles coarse crumbs.
10. Sprinkle the crumb mixture evenly over the top of the cake batter.
11. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
12. Allow the cake to cool in the pan on a wire rack before slicing.

One tip from my grandmother’s kitchen: be patient and let the cake cool completely before serving. The flavors meld beautifully as it rests.