Preheat oven to 375° F. Grease and flour a light-colored 9″ square pan and set aside.
For the cake: In a small bowl, sift together flour, baking powder, and salt; set aside.
In a large bowl, cream together butter and sugar until light and fluffy. Add egg and milk, and then beat until smooth. Add the sifted dry ingredients and mix until just combined. Gently fold in the blueberries and then spread into the prepared pan.
For the crumb topping: In a small bowl, mix together sugar, flour, cinnamon, and salt. Then cut in butter with a pastry blender or fork, until coarse crumbs form. Sprinkle the crumb topping over the cake batter.
Bake for 35 to 40 minutes, until nicely golden brown. Remove from oven and let cool on a cooling rack.
