Preheat oven to 180 degrees C. Grease and flour a 20cm cake tin.
Cake: Sift together the flour, baking powder and salt. Set aside.
In a large bowl, whisk together the oil, sugar and egg. Stir in the flour mixture alternately with the milk, mixing just until incorporated. Fold in the blueberries. Pour batter into prepared cake tin. Cover with streusel crumb topping.
Streusel Crumb Topping: In a bowl, combine flour, cinnamon and sugar. Cut in the butter until mixture resembles coarse crumbs.
Bake in the preheated oven for 45 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.