Preheat the oven to 325F. Oil or line with parchment paper a 9x13-inch baking dish.
In a large bowl, combine the dry ingredients, mixing well.
Make a well in the middle and add the applesauce, blueberries, lemon juice, and vanilla. Stir gently, just until the flour is no longer dry.
Smooth the batter into the pan and bake until a toothpick inserted in the middle comes out clean, about 40 minutes.
Remove from the oven and allow to cool before slicing into 12 pieces and eating.
Sprinkle with confectioner's sugar if desired.