In the bowl of a stand mixer fitted with the paddle attachment, use the back of a wooden spoon or a rubber spatula to press down on the room temperature cream cheese to smooth it out and remove any lumps. Add the butter and begin mixing it with the blueberry powder, salt and vanilla until light and fluffy - about 4 minutes.
Add the powdered sugar and mix to combine. Add milk, if not using the cream cheese, just enough to get to a smooth consistency. Place in an airtight container and set aside (over the counter, or if keeping overnight, store in the fridge then let come to room temperature before piping).
Make the cupcakes
In a small pot, add the blueberries and cook over medium low heat for about 10-15 minutes. They will soften and burst, use the back of a spoon to press them to release their juices. Cook until all the berries are completely soft, most have burst and it’s getting a bit thick. Press through a fine mesh sieve (you should have about 70g or ¼ cup puree), and set aside to cool. Discard the skins.
Preheat oven to 375 F. Line a one dozen cupcake tin with liners.
In a bowl, rub the zest with the sugar, if using. Beat or whisk together the sugar and eggs until very light and fluffy, at least 3-5 minutes. Add the oil, pouring in slowly, and beating as you go.
Stir in the blueberry puree. Add the flour and dry ingredients to the mix, and the kefir. Mix to just combine.
Divide batter between muffin cups, you have enough to make 12 cupcakes. Bake for about 16-18 minutes, until the tops spring back when pressed.
When cupcakes are cool you can frost them with the blueberry buttercream.
Video
Notes
For the blueberry powder, use one 1.2 oz bag (34g) and split the ground powder between the cake and buttercream.