Heat oven to 375 degrees. Butter an 8-inch-square baking pan.
Beat butter in mixer bowl until creamy. Gradually beat in sugar, then beat until light and fluffy. Add egg, vanilla, and orange zest and mix until blended. Stir in milk.
Mix flour, baking powder, and salt; stir gently into butter mixture.
Pare nectarines. Cut into 1/2-inch-thick slices, then cut slices crosswise into thirds. Toss nectarines with lemon juice in mixing bowl. Add blueberries and combine. Fold fruit into batter. Pour into prepared pan and smooth top.
Mix flour, granulated and brown sugars, butter, and cinnamon with pastry blender until mixture resembles coarse crumbs. Sprinkle over batter.
Bake until toothpick inserted into center comes out clean, 30-35 minutes. Cool 15 minutes. Cut into 9 squares and serve warm.