Preheat oven to 350 degrees F. Lightly grease or spray a 13 x 9-inch pan.
For the crumbs, mix together the sugar, flour and lemon zest. Melt butter over low heat in a saucepan. Add the lemon extract. Add the flour mixture to the melted butter mixture and stir until crumbs form, breaking up large clumps. Crumbs should be about the size of peas or a bit larger. Set aside.
In a large bowl with electric mixer, cream butter and sugar until light and fluffy.
Add eggs, one at a time, beating well after each addition. Blend in the sour cream and vanilla.
Mix together the flour, salt, soda and baking powder. Add to the creamed mixture, beating ONLY until flour is incorporated. Fold in blueberries.
Turn into prepared pan, spreading batter evenly in pan. Top with lemon crumbs.
Bake at 350 degrees F for 30-40 minutes, or until a toothpick inserted in the center comes out clean.
Remove from oven to rack to cool. Cool about 30 minutes if you wish to serve the cake warm. Otherwise, cool completely.