Note: Rolling and shaping with the filling may be a bit of messy process, and that’s OK! Just keep a damp towel handy. For a less sticky filling, heat blackberry filling in a small saucepan over medium heat. In a small bowl, combine ½ teaspoon cornstarch and ½ teaspoon water, and whisk into blackberry filling. Bring to a boil, and cook until thickened, about 1 minute. Let cool completely.