Preheat oven to 350 degrees.
For the cake, cream the butter and sugar until combined and light. Add eggs 1 at a time, then vanilla, lemon zest and sour cream; beat until combined. 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.
For the streusel topping, 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.
Butter and flour a 9-inch round baking pan. Spoon batter into 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.