Cream margarine and sugar; add egg, plain yogurt and vanilla. Add dry ingredients, lemond rind and blueberries. Bake at 325°F for 45 minutes to one hour. Sauce: Heat milk in microwave until hot but not boiling. Melt butter in microwave for 30 seconds. Combine brown sugar, flour and melted butter. Whisk into milk mixture until well blended and no lumps remain. Return to microwave and heat slowly, 1 min. at a time, stirring after each minute until mixture comes to a full boil. When ready to serve, reheat in microwave, adding a little more milk if too thick.