In a large mixing bowl, combine the sugar and eggs and whisk to combine. (You also can use an electric mixer.)
Add the lemon juice, lemon zest, Greek yogurt, melted butter and vanilla. Whisk to combine.
Sift in the flour, baking powder and salt. Mix until just combined.
Fold in the blueberries. Fold in the walnuts, if using.
Pour mixture into a 13x9-inch baking pan that's been lined with parchment paper (or sprayed with cooking spray).
Bake in a preheated 350-degree F oven for about 30 to 40 minutes, or until golden and set. It should pass the toothpick test.
Allow to cool before slicing into bars. Sprinkle with powdered sugar, if desired, before serving.
