Preheat oven to 375°F/190°C. Grease a 9x9x2-in. pan and set aside.
In a large bowl, combine oil, egg and buttermilk. Sift together the flour, sugar, baking powder and salt. Add to the milk mixture; blend well. Pour cake batter into greased pan and spread blueberries evenly over batter.
In another bowl, mix together all topping ingredients until well combined. Sprinkle evenly over the batter.
Place pan in preheated oven and bake 40 - 45 minutes or until toothpick inserted and removed from the center comes out clean.
Let stand 5 minutes. Remove coffeecake from baking pan and cool on a rack before slicing for service.
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