Preheat Oven & Prepare Ingredients: Preheat oven to 325°F (163°C). In a bowl, whisk together cake flour, half of the sugar, baking powder, and salt.
Mix the Wet Ingredients: In another bowl, whisk egg yolks, mashed blueberries, vegetable oil, milk, vanilla extract, and lemon zest until smooth.
Make the Meringue: In a clean bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add the remaining sugar and continue beating until stiff peaks form.
Combine Batter & Meringue: Gently fold the egg white mixture into the cake batter using a spatula, being careful not to deflate the mixture.
Bake the Cake: Pour batter into an ungreased 9-inch tube pan and bake for 50-55 minutes, or until a toothpick inserted in the center comes out clean.
Cool & Serve: Invert the cake pan and let it cool completely before removing. Dust with powdered sugar and garnish with fresh blueberries or whipped cream.