In a medium bowl, combine flour, sugar, cinnamon, and salt. Whisk to combine.
Cut in cold butter using a pastry cutter or fork until coarse crumbs appear. I use a fork or my hands.
Preheat oven to 425°F. Grease and line, optional, a 12-cup muffin pan with liners.
Whisk together flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, whisk together sugar, brown sugar, butter, oil.
Add eggs, sour cream, milk, and vanilla. Whisk until smooth.
Add dry ingredients to wet ingredients. Mix until just combined. Do not overmix. A few streaks of flour are perfectly fine.
Gently fold blueberries into batter. Avoid crushing them.
Rest batter for 20 minutes.
Fill each muffin cup nearly to the top.
Generously top each muffin with streusel.
Bake at 425°F for 7 minutes. Without opening the oven, reduce temperature to 350°F.
Bake another 15. Insert a toothpick for doneness. You want a few moist crumbs to come out on the toothpick. If not, bake in 2-minute increments—no more than 18 minutes.
Cool in pan for 5 minutes. Transfer to a cooling rack.