Preheat oven to 350°F (175°C).

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, melted butter, lemon juice, orange rind, orange juice, and vanilla extract to the bowl.

Whisk until smooth.

In a separate bowl, 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 the flour mixture.

Fold in fresh blueberries.

Pour into an 8-inch square baking pan coated with cooking spray.

Place the baking pan in a larger baking pan.

Add hot water to the larger pan to a depth of 1 inch (water bath).

Bake at 350°F (175°C) for 40 minutes or until the cake springs back when lightly touched in the center.

Sift powdered sugar over the top.

Serve warm or at room temperature.