Preheat oven to 400 degrees and coat a muffin tin with cooking spray. Set aside.
Combine the flour, sugar ,baking powder in a bowl.
Combine the milk, egg, oil, and vanilla in a separate bowl and whisk until combined.
Pour the milk mixture into the dry and lightly mix just until no dry spots remain-- the more more you mix the tougher the muffin will be.
Gently fold frozen or fresh blueberries into batter and immediately scoop into the muffin tin.
Bake for 15-20 minutes until the center of each muffin is set. You'll know it's set when you gently press on the top of a muffin and it springs back instead of sinking. Remove from the oven, allow to cook for 5 minutes and remove from muffin pan.
If you don't have self-raising flour on hand, combine 2 C All Purpose flour, 1 teaspoon salt, and 1 tablespoon of baking powder in a small bowl. Whisk until well combined and measure out the amount you need for the recipe.
I used frozen wild blueberries for the muffins in the pictures and video. I like the wild blueberries because they are small and are great for muffins but they do bleed ALOT. Meaning that if you're not careful your batter will turn purple incredibly quick. If you'd like to use these blueberries make sure they are VERY frozen and just barely fold them in enough to get them somewhat evenly through the batter. Standard frozen blueberries will bleed but not nearly as much and fresh blueberries don't bleed at all. Use what have!