Preheat oven to 350°F (175°C) and grease a 9-inch round or square cake pan.
In a bowl, whisk together flour, baking powder, baking soda, salt, and espresso powder.
Beat butter and sugar together until creamy, then add eggs and vanilla, mixing until smooth.
Add sour cream and dry ingredients alternately, mixing until just combined.
Fold in blueberries gently, then transfer batter to the prepared pan.
For streusel, combine oats, brown sugar, flour, butter, and cinnamon in a bowl.
Sprinkle streusel over the cake batter.
Bake for 35-40 minutes or until a toothpick inserted comes out clean.
Let cool on a rack before serving.