Prepare the Streusel and baking pan: Place all the streusel ingredients in the bowl of a food processor and blend until it resembles breadcrumbs. Set aside in the fridge to keep cold until ready to use.
Preheat the oven to 325°F (165°C). Grease and line the base and sides of a 9-inch springform cake pan.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Cream Butter and Sugar: Mix the butter and sugar on medium-high until very light and fluffy in a stand mixer bowl fitted with the paddle attachment.
Add Eggs and Vanilla: Scrape down the sides and bottom of the bowl, then add the eggs and vanilla. Mix for another 30 seconds until just combined.
Combine Wet and Dry Ingredients: Add the flour mixture and sour cream alternately in three additions, beginning and ending with the flour. Pour the batter into the prepared cake pan and refrigerate for about 20 minutes to firm up.
Add Fruit and Streusel: Arrange the sliced nectarines and blueberries on top of the batter. Sprinkle with about ½ cup of the streusel topping.
Bake: Bake for 35-40 minutes, or until the cake is golden on top, the blueberries have burst, and the top springs back when touched lightly.