Preheat oven to 350 F (175 C). Line a muffin tin with baking cups and set aside.
In a large bowl sift flour, then add salt, baking soda and baking powder and give it a stir.
Whisk egg, then add sugar and mix well. Add butter and mix until well combined. Add vanilla and sour cream. Mix until homogenous.
To the bowl with flour mixture add frozen blueberries and toss them lightly to cover with flour, so they don't sink while baking.
Slowly fold in the egg, sugar, and sour cream mixture into the flour mixture, using a spatula. Don't be surprised, this muffin batter will be pretty thick.
Using an ice cream scoop, distribute batter into baking cups. To give muffins a nicer look, you can top with 2-3 additional frozen blueberries.
Bake for 35-38 minutes, until toothpick inserted in the middle, comes out clean (there might be some crumbs on the tooth pick, but no batter).
Let them cool for 10 minutes inside baking tin. Then remove them and set on a cooling rack to cool completely.
The muffins can be stored in a box at room temperature for up to 3 days and in the freezer for up to 2 months.