In a medium sauce pan over medium heat, place blackberries, sugar, water, lemon juice, and salt. (Ingredients listed under "Blackberry Filling". Bring to a simmer while stirring together. Continue for 3-4 minutes until blackberries breakdown and juices release.
Stir in corn starch and stir for another 1-2 minutes until mixture thickens somewhat. Remove from heat and allow to cool.
Preheat oven to 400 degrees and line a sheet pan with parchment paper.
Sprinkle flour on a large cutting board or work surface. Place one sheet of thawed pastry onto floured surface and cut into 4 equal squares.
Spoon 2 TBS blackberry filling (drain excess juice from measuring spoon) onto one side of square. Brush egg mixture along all edges of the square. Fold over the pastry at a diagonal to make a triangle shape. (Make sure the juice doesn't go into the egg wash, if possible.) Using a fork press edges together to seal.
Transfer the pastry onto the prepared sheet pan and complete the other 7 turnovers.
Bake in preheated oven for 18-20 minutes until golden brown and puffed up. Remove from oven and allow to rest for a few minutes.