Make the cupcakes:
Preheat the oven to 350°F. Line two standard muffin tins with 15 paper liners.
In a medium bowl combine the flour, baking powder, baking soda, and salt.
In a large bowl, use an electric mixer on medium-high speed to beat the butter and sugars until fluffy and very well combined, about 3 minutes. On medium-low speed , add the eggs and yolk, one at a time, until combined. Add the buttermilk, lemon zest, and lemon juice. Add in the flour mixture, stirring just until combined.
In a small bowl toss the blueberries with the 2 tablespoons of flour. Add into the batter and fold with a spatula until combined. Be careful not to overmix.
Divide the batter among the prepared muffin tins, filling each 3/4 full. Bake for 15 to 20 minutes, or until a toothpick inserted in the center comes out clean. Allow the cupcakes to cool in the pan for 5 minutes before removing to a wire rack to cool completely.
Make the buttercream:
Using a blender, immersion blender or food processor, puree the blueberries. Pass the pureed blueberries through a fine mesh strainer and into a medium bowl to remove the skins. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, mix together the sugar and butter. Mix on low speed until well blended and then increase the speed to medium and beat for another 3 minutes. Add the salt and vanilla and beat on medium for 1 minute.
Remove half of the buttercream to the bowl with the blueberry puree and stir until well combined. If the buttercream is too thin, add more powdered sugar to thicken. If desired, add a few drops of purple food coloring to enhance the blueberry color.
To the plain buttercream, add the cream and beat until well combined.
In a large piping bag fitted with a decorative tip (such as the Ateco #846), use a spoon to scoop the plain frosting into one section of the bag, using your finger on the outside of the bag to help scrape it off. Repeat with the blueberry frosting. Ensure you’re aligning the frosting so it starts at the same place in the bag. Don’t worry if the colors touch each other.
Squeeze the bag to slightly blur the colors together for a watercolor type effect. Twist the bag and squeeze out a test pipe to ensure the colors are coming out evenly before piping onto your cupcakes. Garnish frosted cupcakes with lemon zest.
Serve or store at room temperature for up to 1 day, or in the fridge for up to 3 days. Let come to room temperature before serving.