Let eggs stand at room temperature for 30 minutes. Sift flour and baking powder; set aside.
Place oven rack in lowest position. Preheat oven to 325°. In a large bowl, beat yolks until slightly thickened. Gradually add sugar, beating until thick and lemon-colored. Blend in orange juice. Add dry ingredients to yolk mixture until well blended
Add cream of tartar to egg whites; with clean beaters, beat on high until stiff but not dry. Fold a fourth of the egg whites into batter, then fold in remaining whites.
Gently spoon into an ungreased 10-in. tube pan; smooth the top. Bake 50-55 minutes or cake springs back when lightly touched. Immediately invert the pan; cool completely.
For blueberry topping, in a large saucepan, combine sugar, cornstarch and orange zest. Stir in orange juice until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat. Stir in blueberries.
For sour cream topping, in a large bowl, combine sour cream, confectioners' sugar and vanilla.
Run a knife around side and center tube of pan. Remove cake to a serving plate; cut into slices. Serve with warm blueberry topping and the sour cream topping. Garnish with orange zest if desired.