Preheat your oven to 300F degrees. Spray a bundt pan with nonstick cooking spray and dust with flour. Shake to coat sides well and tap to release excess flour from the pan. Set aside.
In a bowl, whisk together the flour, baking soda, and salt. Set aside.
In a small bowl combine the streusel ingredients and set aside.
In a large mixing bowl with an electric mixer or in the bowl of a stand mixer, cream together the sugar and butter until light and fluffy.
Add the eggs to the butter mixture one at a time, beating well after each addition.
Reduce the speed and add in the vanilla extract and the sour cream. The batter will look a little curdled, but this is okay.
Slowly add in the flour mixture, a little at a time to make sure that it's smooth and well incorporated.
Gently fold the blueberries into the batter and then scoop out ½ of the batter and place it in the bottom of the prepared bundt pan. Sprinkle half of the streusel mixture over the batter as evenly as you can.
Scoop another ½ of the cake batter and spread it over the top of the streusel layer.
Bake for 1 hour 10 minutes to 1 hour 20 minutes, or until the cake is a golden color and a toothpick can come out clean. Please note that depending on how wet the streusel is the cake tester might come out looking wet despite the cake being fully cooked. If needed, press slightly on the top of the cake to get a feel for how wet it is. If there's a little spring back when pressed, the cake should be cooked through.
Remove the cake from the oven and let it rest in the pan for 15 minutes before flipping it over to cool on a wire baking rack without the pan until cooled. Dust with powdered sugar if desired, serve and enjoy.
Notes
Use butter for the streusel topping, not margarine or butter spread, to avoid sogginess in the cake.You can add extra blueberries to the recipe, ideally when applying the first layer of streusel to ensure an even distribution.When stored in an airtight container at room temperature, the coffee cake will last for up to a week.You can also refrigerate the cake, but it might dry out due to the cold; it's best enjoyed warm.To freeze the coffee cake for up to 3 months, allow it to cool completely, wrap it in plastic wrap, and store it in an airtight container. Thaw it on the counter overnight before serving.