In a medium bowl, whisk together the flour, baking powder and salt. These are your dry ingredients--set them aside for now.
In the bowl of your mixer, cream the butter and sugar together on medium-high for 2-3 minutes, or until light and fluffy. Scrape down the sides of your bowl and add the eggs, one a time, followed by the vanilla. Continue beating for another minute until creamy.
Add the milk and sour cream and mix until combined. Then, add all of the dry ingredients at once and mix on the lowest speed possible or by hand until they are just combined. A few lumps are okay--do not over mix.
Toss the blueberries with flour and then gently fold them into the batter with a flexible spatula.
Transfer the batter to a large bowl, cover, and chill in the fridge for at least an hour, or overnight.
When you're ready to bake, preheat the oven to 375°F and line 12 muffin cups. Divide the batter evenly between the 12 cups.
Make the crumb topping by mixing the flour, brown sugar, cinnamon, and melted butter together in a small bowl until a dough comes together. Crumble a little bit on top of each muffin.
Bake for 18-20 minutes. You can test if the muffins are done by inserting a toothpick, which should come out clean or with a few moist crumbs clinging to it. Let the muffins rest in the pan for 10 minutes to fully set before serving. Enjoy!