HEAT THE OVEN: Preheat your oven to 325 degrees. Spray three 8-inch round pans with nonstick spray, line the bottom of each pan with parchment paper and spray again. Set aside.
MAKE THE PUREE: In a medium saucepan, cook the blueberries over medium-low heat until the berries soften. Transfer berries to a blender or food processor and pulse until smooth. Should make 1 cup puree. Set aside.
MIX THE INGREDIENTS: In a medium bowl whisk together the cake flour, baking powder, and salt. Set aside.
In a bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high for about 2 to 3 minutes, until lighter in color.
With the mixer in medium speed, add the egg whites and egg, one at a time. Make sure to scrape down the sides and bottom of the bowl between additions. Once all of the eggs are added, turn the mixer to medium-high and beat for an additional 2 minutes.
Stir in the vanilla extract and blueberry emulsion.
With the mixer on low, alternately add the dry ingredients with the blueberry puree in three additions, starting and finishing with the dry ingredients.
NOTE: I left my batter naturally colored by the emulsion and puree. For more of a purple or blue, use a drop of food coloring.
BAKE THE CAKE: Evenly divide the batter between the three pans (about 15 ounces of batter in each) and bake for about 25 to 30 minutes, or until a toothpick inserted in the center of the cake comes out with a few moist crumbs on it.
Let the cake layers cool on racks for 10 minutes before inverting onto greased wire racks to cool completely.
Wrap each cake layer with plastic wrap and chill in freezer for at least an hour before using. The cake layers can be stored for up to a few days wrapped once in plastic wrap and frozen. To store longer than a few days, wrap twice in plastic wrap, then in foil, and seal in a zip-lock bag.