Preheat oven to 375 degrees F. Combine butter and sweetener in a mixing bowl. Use a power mixer to cream them together. This should be very quick, much quicker than it would be with granulated sugar.
Add eggs one at a time to the mixing bowl, with mixer on low. While still running the mixer, add vanilla extract and milk.
Add flour, baking powder, and salt. Mix until fully integrated, scraping down the sides with a spatula as needed.
Smash 1/2 cup of blueberries, and mix in with batter.
Carefully fold in remaining blueberries, by hand, taking care not to break any.
Place paper liners in muffin tin. Place better in each using an ice cream scoop.
Bake for 25 to 30 minutes. Muffins are done when a toothpick can be inserted and removed cleanly.
Remove muffins from muffin tins, and place on a cooling rack. Allow to cool for a few minutes, and then try to stop yourself from eating all of them.