Combine sugar and cornstarch in a 3-quart saucepan; gradually whisk in Blackberry Juice. Whisk in eggs and egg yolks. Bring mixture to a boil (5 to 6 minutes) over medium heat, whisking constantly.
Cook, whisking constantly, 1 to 2 minutes or until a pudding-like thickness. Remove from heat, and whisk in butter. Cover, placing plastic wrap directly on curd, and chill 8 hours.