In a medium mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add the 6 tablespoons sugar, beating until stiff peaks form. Pour berry mixture into pie crust. Carefully spread custard evenly over blueberry mixture. Spread meringue over custard, spreading to edges to seal. Bake the meringue-topped pie in a 350 F or 176 C oven for 15 minutes. Cool on a wire rack. Store in the refrigerator. Makes 8 servings.