Preheat oven to 350℉. Spray a 9x9 inch pan with non stick cooking spray.
With a hand mixer or whisk, combine the butter, lemon zest as well as the juice from the lemon, and sugar until well combined and creamy.
Add your egg and vanilla to the mixture, and continue to mix until all the ingredients are well combined.
Whisk in your flour (reserving ¼ cup of it for the blueberries), baking powder, and salt, until the wet is fully combined with the dry ingredients.
Mix in the buttermilk with whisk.
In a separate bowl, toss the fresh blueberries with the ¼ cup flour, until they are all lightly coated.
Fold the blueberries into the batter, until fully combined.
Spread the cake batter into your prepared 9x9 inch pan, evenly.
Place in oven and cook for 35-40 minutes.
Remove from oven and let it cool for about 10 minutes.
Cut and serve
Notes
For this recipe, I would not recommend using a cake mix, because of the ingredients in the cake mix, combined with these ones, might make a bit of a weird texture. It's also a lot thinner, so your cake won't be as moist.