Preheat oven to 425º F. Spray muffins tins with non-stick cooking spray. Set aside.Pour milk into measuring cup. Add lemon juice to milk and allow to sit while preparing other ingredients.Pour buttermilk cornbread mix into medium bowl. Add sugar, blueberries, and lemon zest. Slowly pour milk mixture into bowl with dry cornbread mixture and stir slowly and gently until well combined.Fill muffin tins ¾ full and bake for 15 minutes or until golden brown and cooked through.Allow to cool slightly before removing from muffing tin. Serve warm.