Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and 2/3 cup sugar in a large bowl; add buttermilk and next 6 ingredients, stirring with a whisk until smooth.
Beat egg whites with a mixer at high speed until soft peaks form. Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold egg white mixture into flour mixture. Fold in blueberries. Pour into an 8-inch square baking pan coated with cooking spray. Place pan in a larger baking pan; add hot water to larger pan to a depth of 1 inch. Bake at 350° for 40 minutes or until cake springs back when lightly touched in center. Sift powdered sugar over top. Serve warm or at room temperature.