On a lightly-floured work surface, roll out one of the discs of chilled dough (keep the other one in the refrigerator). Roll it into a rectangle about 1/8 inch thick and 9×12 inches in size. Trim the sides as needed. Using a pizza slicer or knife, cut 9 rectangles, each measuring 3×4 inches. Using a ruler will make this process much easier and more accurate.
Place the 9 rectangles onto a baking sheet lined with parchment paper. Place the baking sheet in the fridge.
Roll the second disc out into a rectangle, and cut into 9 even rectangles like you did with the first disc. Put the rectangles on a parchment paper-lined baking sheet and place in the refrigerator.
Remove the first baking sheet of rectangles from the refrigerator. Brush egg wash along the border of each rectangle. This will act as the "glue". Place a heaping Tablespoon of the blackberry filling into the center of each rectangle, spread it around, leaving around 1/2 inch of space on the edges.
Remove the second baking sheet of rectangles from the refrigerator and place each one on top of the filling-topped rectangles. Use your fingertips to press firmly around the pocket of filling, sealing the dough well on all sides. Crimp the edges with a fork.
Place the filled pop tarts in the refrigerator for at least 20 minutes. Preheat the oven to 400ºF.
Once pop tarts have chilled, lightly brush the tops with egg wash. Bake for about 15-17 minutes, or until they’re golden brown. Remove from the oven and allow to cool completely.