Preheat oven to 350 degrees. Butter nine 2 1/2-by-4-inch baking pans, and dust with flour, tapping out excess.
Combine 1/2 cup sugar and 1/2 cup lemon juice in a small saucepan over medium heat. Simmer, stirring occasionally, until sugar dissolves and syrup is thickened, about 4 minutes. Let stand while making cakes.
Meanwhile, combine heavy cream, remaining 2 tablespoons lemon juice, and the vanilla in a bowl. Whisk together flour, baking powder, baking soda, and 1/4 teaspoon salt in another bowl.
Beat butter, lemon zest, and remaining cup sugar with a mixer on medium speed until pale and fluffy. Reduce speed to low, and add eggs, 1 at a time, beating well after each addition. Beat in flour mixture in 3 additions, alternating with cream mixture, beginning and ending with flour. Gently fold in blueberries using a rubber spatula.
Divide batter evenly among pans. Bake on a rimmed baking sheet until a toothpick inserted into the center of each comes out clean, about 30 minutes.
Transfer pans to wire racks, and brush tops of cakes with lemon syrup. Let stand for 15 minutes. Turn out cakes from pans, and brush all over with remaining syrup. Let cool completely on wire racks.
Cakes can be stored in an airtight container for up to 2 days.
To make a large loaf instead of several little ones, use a 5-by-9-inch loaf pan, and bake for 55 minutes.