Preheat oven to 400 degrees F. Lightly coat a baking sheet with oil or cooking spray. Place cream cheese in a small bowl and stir until completely smooth. Separate the egg yolk from the egg white. Place the egg white in a small bowl and add the egg yolk to the cream cheese. Add the sugar to the cream cheese and stir until combined.
Lay the sheet of puff pastry on a flat surface and cut into nine small squares. Spoon a tablespoon of the cream cheese mixture down the center of each square from one corner to the opposite corner. Press the blueberries into the cream cheese. Fold the sides into the center, wet the points with a little water and press tightly to seal. (Press the sides tightly together or they will open while they bake).
Place the turnovers on the baking pan, brush with the egg white and sprinkle lightly with the sugar. (If you are making these ahead, you can cover the pan with plastic wrap and refrigerate them for up to 4 hours before baking.) Bake for 15-17 minutes until golden brown. Remove from oven and serve warm.
Megan Carle, a senior at Arizona State University and co-author with sister Jill of College Cooking (Ten Speed Press, 2007), recommends these three-bite treats served warm as the perfect end to an "Impressing Your Date" meal.
Courtesy College Cooking (Ten Speed Press, 2007) by Megan and Jill Carle | Photo credit Jessica Boone