Combine blueberries, honey, lemon juice, and cornstarch mixture in a small saucepan. Bring to a simmer over medium heat and cook until blueberries are broken down and thickened, stirring occasionally. Allow filling to cool.
Preheat oven to 400F and line a large sheet pan with parchment paper. Set aside.
Remove pastry dough from refrigerator and turn onto a floured surface. Keep dough folded into thirds and roll into a 10-inch square. Use a knife or pizza cutter to cut into 9 even squares.
Place about 1/2-1 tablespoon blueberry filling in each square and fold over into a triangle shape. Use a fork to seal and crimp edges. Place each turnover on prepared sheet pan. Chill turnovers in freezer 15 minutes.
Remove turnovers from freezer, brush with egg white mixture, and sprinkle with coarse sugar. Bake turnovers at 400F 15-20 minutes until golden brown.
Allow turnovers to cool on sheet pan 10 minutes, then transfer to a wire cooling rack. Enjoy!