Add the blackberries, sugar and 1/4 cup water to a saucepan. Bring to a boil over medium-high heat and cook until the juices thicken slightly, 4 to 5 minutes.

In a small bowl, make a slurry by stirring together the cornstarch and 2 tablespoons water until smooth. Add the slurry to the berries, return to the boil and cook for another 1 to 2 minutes. Turn off the heat and cool the mixture.

You can chill this overnight until you are ready to make the danishes,