Preheat oven to 375 degrees (F).
Grease an 8-inch or 9-inch square pan.
In a bowl, blend the flour, baking powder, and salt.
In a separate bowl, beat together the butter, 3/4 cup sugar, egg, vanilla, and lemon zest.
Alternately add the milk and the flour mixture to the sugar/butter mixture.
Add blueberries, stirring only enough to blend.
Pour batter into pan.
In a small bowl, blend the white and brown sugars, flour, and cinnamon.
With a fork or pastry blender, cut in butter until mixture is crumbly.
Sprinkle evenly over the cake batter.
Bake at 375 degrees (F) for 40 to 45 minutes until a toothpick inserted in the center comes out clean.
Cool on wire rack, then serve right from pan.