1Heat oven to 350°F. Lightly spray 13x9-inch pan with cooking spray.
2In large bowl, beat butter and brown sugar with spoon until well blended. Add flour, oats, baking powder and salt; stir until mixture is crumbly. Press half of mixture in bottom of pan.
3In small bowl, stir together condensed milk, lemon peel and lemon juice. Pour over crust in pan. Scatter blackberries over lemon mixture. Sprinkle remaining crumb mixture over top.
4Bake 20 to 25 minutes or until golden brown. Cool 30 minutes. Refrigerate until completely cooled, about 45 minutes. For bars, cut into 6 rows by 3 rows. Store in refrigerator.