In a medium bowl, whisk together the flour, cornmeal, sugar, baking powder, baking soda and salt. In a small bowl, whisk the buttermilk with the egg yolk. In a medium bowl, beat the egg white until firm peaks form. Add the buttermilk mixture to the dry ingredients and stir gently with a wooden spoon until evenly moistened; there will still be some lumps. Fold in the beaten egg white and then the blueberries.