In a bowl, cream butter, shortening and sugar. Beat in eggs, one at a time. Add extracts. Combine flour and baking powder, add creamed mixture and mix well. Spread 2/3 of the batter in a greased 15x10x1 baking ban.
For filling, combine flour, cornstarch and tapioca in a large bowl. Add 1/2 cup blueberries, toss to coat. Pour evenly over batter in pan. Drop remaining batter by rounded tablespoonfuls over filling . Bake at 350 degrees for 40 minutes or until golden brown. Combine glaze ingredients, drizzle over warm cake.