For the streusel, combine the granulated sugar, brown sugar, cinnamon, and nutmeg in a bowl.
Stir in the melted butter and then the flour.
Mix well and set aside.
For the cake, cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment on high speed for 4 to 5 minutes, until light.
Reduce the speed to low and add the eggs one at a time, then add the vanilla, lemon zest, and sour cream.
In a separate bowl, sift together the flour, baking powder, baking soda, and salt. With the mixer on low speed, add the flour mixture to the batter until just combined. Fold in the blueberries and stir with a spatula to be sure the batter is completely mixed.
Spoon the batter into the prepared pan and spread it out with a knife.
With your fingers, crumble the topping evenly over the batter.
Bake for 40 to 50 minutes, until a cake tester comes out clean.
Cool completely and serve sprinkled with confectioners' sugar.