Preheat the oven to 350°F. Butter and line a 9x5-inch loaf pan with parchment paper, ensuring a 1-inch overhang.

In a large bowl, whisk together all-purpose flour, whole-wheat flour, baking powder, and salt.

Grate both lemons to obtain about 1/4 cup of zest; place it in a medium bowl. Juice 1 lemon for 2 tablespoons and add it to the bowl. Thinly slice the remaining lemon, removing any seeds.

Combine cottage cheese, eggs, and 3/4 cup sugar in the lemon mixture, whisking until well combined.

Gently fold the wet ingredients into the dry mixture until just combined. Add blueberries and melted butter, then transfer the batter to the prepared pan.

Top with lemon slices and sprinkle with the remaining 1 tablespoon sugar.

Bake until the top is golden-brown and a toothpick inserted in the center comes out clean, around 1 to 1 hour and 10 minutes. Allow the loaf to cool before slicing.