Preheat the oven to 340°F, butter and flour three 6-inch pans, and use damp cake strips for even baking.
In a large bowl, sift together the dry ingredients, including sugar.
Combine the wet ingredients in a medium bowl.
In a small bowl, toss blueberries and lemon zest with 1-2 tbsp of flour.
Mix the wet ingredients into the dry until just combined, then gently fold in the blueberry and lemon zest.
Divide the mixture evenly among the cake pans.
Bake for approximately 30 minutes or until the centers spring back when touched.
Cream the butter in a stand mixer, then sift in the confectioners' sugar in batches, scraping down the bowl and mixing until light and uniform in color.
Gradually add lemon juice to reach the desired consistency and taste.
Take about 1/2-3/4 cup and mix it with blueberry reduction for the desired flavor.
Transfer blueberry buttercream and an equal amount of lemon vanilla buttercream into piping bags, combine them in one bag, and snip off the tip for piping.