Add blackberries, sugar and lemon juice to a non-reactive pan. Cook uncovered, stirring occasionally, until the fruit begins to soften (about 8 minutes).
2 cups fresh or frozen blackberries (10 ounces), ¼ cup granulated sugar, 1 teaspoon lemon juice
Add corn starch and cardamom and continue to cook until the fruit is broken down (about 3 minutes longer).