Spread blueberries on bottom of a greased (sprayed with cooking spray) 9x13x2-inch baking dish. Sift together flour, baking powder and salt (if using). Reserve. In a bowl, cream together 1 1/2 cups white sugar and 2/3 cup margarine. Combine yogurt and orange juice. Reserve. Alternate additions of flour mixture and yogurt mixture to creamed mixture. Spread over blueberries. Combine 1/2 cup white sugar and corn starch. Sprinkle over batter. Pour boiling water and combine with 2 tbsp lemon juice over batter. Do not stir. Bake at 400 degrees for 40 - 50 minutes, until skewer inserted in centre comes out clean. Best served warm with whipped cream or ice cream.