Once chilled, fillings and sauces thickened with cornstarch will loosen when stirred. So, when ready to use, don't stir. Just spoon the curd directly into tart shells, or spoon and spread over cake layers.
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.