Preheat a grill to medium. In a large bowl, stir together 1 cup ricotta cheese, 1/4 cup sugar, 2 tablespoons cream cheese, 1/4 teaspoon salt and the grated peel of 1 lemon; fold in one 6-ounce container blueberries. Spoon 2 tablespoons of the blueberry mixture onto each of 8 slices sandwich bread with the crusts trimmed; top each with another trimmed bread slice. In a shallow bowl, beat 2 eggs with 2 tablespoons water. Working with 1 sandwich at a time, dip all 4 edges quickly into the egg wash and gently pinch to seal. Brush each turnover with melted butter and sprinkle with some sugar. Arrange on an oiled cooking grate and grill, covered and turning once, until golden, about 7 minutes. Serve warm.