Preheat oven to 375. grease an 8 x 8" pan. Drain blueberries from mix; rinse with cold water; drain again and spread on paper towels to drain. For base, empty muffin mix into a med. Bowl. Break up any lumps. Stir in brown sugar; drizzle with butter. Toss with a fork until crumbs form. Reserve 3/4 c. Of mixture; press remaining crumbs into bottom of pan. For Filling: Combine jam and lemon juice. Fold in blueberries. Spread over base. Sprinkle with reserved crumbs. Bake for 25-30 min. Or until golden brown and jam bubbles slightly. Loosen from edge of pan; cool. Cut into bars. Sift with confectioners' sugar.