In a medium-sized mixing bowl or bowl of a stand mixer, mix together sugar, cake flour, baking powder, baking soda, salt, and lemon zest.
Add butter and mix on medium-low speed for three minutes. Because there is so little butter, you'll end up with a very fine crumb texture.
In a small mixing bowl, whisk together eggs, sour cream, oil, and vanilla extract until smooth.
Add the egg mixture to the flour mixture and beat on medium speed until just combined.
Slowly add milk and mix on low speed until just combined. The batter will be liquid. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)
Gently fold in blueberries.
Fill cupcake liners just over 1/2 full.
Bake for 14 minutes and then test to see if they are done. They are done when a toothpick comes out without wet batter stuck to it. The cupcakes should appear white with specks of vanilla bean. They should not turn a golden brown. If they are not done, test again in two minutes. If they are still not done, test again in another two minutes.
When the cupcakes are done, remove them immediately from the tins and leave them on a cooling rack (or just on your counter if you don't own a cooling rack) to cool.
Frosting Instructions
Heat blueberries in a small pot on medium-low heat, stirring periodically for about five minutes or until they soften and start to release their juice.
Push the cooked blueberries through a fine sieve to get a smooth blueberry sauce.
Beat butter on high speed with an electric mixer for about three minutes until light and airy.
Mix in powdered sugar a little bit at a time.
Add salt and continue to beat for another minute. If desired, add more powdered sugar to make the buttercream frosting stiffer.
Remove half of the frosting and place in a separate bowl.
Add 3 teaspoons of cooled blueberry sauce to the frosting still in the electric mixer bowl and mix until the frosting looks uniformly light purple.
Place some of each kind of frosting into one piping bag. Use a spatula to spread vanilla frosting on one half of a piping bag fitted with a star tip and blueberry frosting on the other half of the bag.
Pipe a frosting swirl onto cooled cupcakes.
Notes
Be careful not to over-mix the batter.
Use room temperature ingredients - everything will mix together more smoothly that way.
If you need to use frozen blueberries, bring them to room temperature before using.