Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a mixing bowl, combine blackberries, sugar, lemon juice, cornstarch, and cinnamon (if using). Stir gently to combine and let it sit for 5-10 minutes to allow the sugar to dissolve and the mixture to thicken.
If using puff pastry, roll it out on a lightly floured surface to about 1/8-inch thick. If making homemade strudel dough, stretch it out until it’s thin and translucent.
Spoon the blackberry filling into the center of the dough, leaving some space on each side.
Fold the sides of the dough over the filling and seal the edges to create a roll or parcel.
Beat the egg with 1 tablespoon of water and brush the top of the strudel with the egg wash to achieve a golden finish.
Bake the strudel for 30-35 minutes, or until the dough is golden and crisp.
Let the strudel cool for a few minutes before slicing and serving. Optionally, dust with powdered sugar before serving.