In a medium bowl, mix 2 cups flour, baking powder, and salt. In a large bowl, with mixer on medium-high, beat 1/2 cup butter until smooth. Add 1/2 cup sugar; beat until light and fluffy.
Reduce speed to medium. Beat in egg until combined. Beat in flour mixture alternately with milk until combined, starting and ending with flour mixture and scraping bowl as needed. Transfer to a greased 9-inch square baking pan.
In a small bowl toss blueberries with lemon juice. Sprinkle over batter.
In another bowl, whisk cinnamon, nutmeg, and remaining 1/3 cup flour and 1/4 cup sugar. Add remaining 4 Tbsp. butter. With pastry blender, blend well. Form into large crumbs; sprinkle over blueberries.
Bake in a preheated 375-degree oven to 50 minutes or until a toothpick comes out almost clean. Cool slightly in pan on wire rack. Cut into squares. Serve warm or at room temperature. Top with vanilla ice cream, if desired. Store, refrigerated, up to 3 days.