Preheat oven to 350 degrees. Make the cake: Coat two 8-by-1 1/2-inch round cake pans with cooking spray. Line bottoms with parchment. Spray parchment; set aside.
Sift together flour, baking powder, and salt into a medium bowl; set aside. Whisk together 1/4 cup sugar, the milk, oil, vanilla, lemon zest, and lemon juice in a large bowl; set aside.
Put egg whites into the bowl of an electric mixer fitted with the whisk attachment. Beat on medium speed until foamy. Raise speed to medium-high. Gradually add remaining 1/4 cup plus 2 tablespoons sugar, beating until stiff peaks form.
Add half of the flour mixture to the milk mixture; whisk until smooth. Fold in remaining flour mixture in 3 batches, alternating with the egg white mixture. Divide batter among prepared pans. Bake, rotating pans halfway through, until a cake tester inserted into centers comes out clean, about 18 minutes. Let cool on a wire rack for 10 minutes. Turn out cakes onto rack. Remove parchment; reinvert. Let cool completely.
Make the lemon curd: Put egg yolks
into a medium heatproof bowl; set aside.
Whisk together lemon zest, sugar, cornstarch,
and salt in a medium saucepan.
Add 1 1/2 cups water and the lemon juice;
whisk until sugar and cornstarch have
dissolved. Bring to a boil over medium-high
heat, whisking constantly. Cook 2
minutes. Reduce heat to medium-low;
gradually whisk one-third of the lemon
mixture into the reserved egg yolks. Pour mixture into pan. Cook over medium heat, stirring constantly, 2 1/2 minutes.
Remove from heat. Stir in vanilla. Pour into a bowl; place plastic wrap directly on surface. Refrigerate until firm and cold, at least 1 hour (up to 1 day).
Fold blueberries into curd; set aside. Trim top and bottom of one cake to be flat. Cut each cake in half horizontally. Set untrimmed bottom layer on a serving plate; spread with 1 cup curd. Repeat with trimmed halves for second and third layers. Top with untrimmed top layer. Dust with confectioners' sugar.