Preheat oven to 400°F. Line two baking sheets with parchment paper or a baking mat. Set aside.
In a bowl, mix the blueberries, sugar, and cornstarch together. Set aside.
Unfold the puff pastry and cut into nine pieces. Take a piece and work the dough into an even square.
Add 1 ½ tbsps of the blueberry mixture to the dough square. Use a brush to brush some egg wash around the edge of half the dough square.
Gently fold the dough in half and seal it with the prongs of a fork. Make sure there are no open parts and the turnover is sealed.
Repeat process with remaining pieces of dough. Place the turnovers on the baking sheets.
Brush the top if each turnover with remaining egg wash.
Bake for 20 minutes. Remove from oven and allow the blueberry turnovers to cool before glazing.