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.
If using frozen blueberries, use without thawing to avoid discoloring the batter.