Preheat oven to 350 degrees.

Make streusel topping: In a medium bowl, stir together flour, brown sugar, and salt. Cut in the butter using your hands or a pastry blender until large, moist crumbs form. Chill until needed.

Coat a 9-inch square baking pan with non-stick cooking spray.

In a medium bowl, whisk together 1 1/2 cups flour, baking powder, baking soda, salt, and allspice.

In a large bowl, beat the butter and granulated sugar with an electric mixer until fluffy. Add the egg and beat well.

Add flour mixture and buttermilk alternately until just combined, starting and ending with the flour. The batter will be very stiff.

In a large bowl, toss the blueberries with remaining teaspoon of flour.

Fold blueberries into the batter; spread into prepared pan.

Sprinkle cake with streusel topping.

Bake until golden brown, 45 to 50 minutes.

Cool completely, then dust with powdered sugar, if desired.