Preheat oven to 375F. Spray a 9x9-inch square baking dish with nonstick cooking spray.
In a medium bowl, combine 2 cups all-purpose flour, 2 teaspoons baking powder and 1/2 teaspoon salt with a fork or whisk until combined.
In the bowl of a stand mixer (or using a handheld electric mixer in a separate bowl) beat together 1/4 cup oil, 1 large egg, 1 cup sugar and 1 teaspoon vanilla extract.
Alternately add the flour mixture and 3/4 cup half and half to the sugar/oil mixture (ending with the flour mixture.)
Batter will be very thick. Finally, stir in 2 cups fresh or frozen blueberries, by hand, just until combined.
Spread batter into prepared baking dish.
To make the crumble topping, combine 1/3 cup sugar, 1/2 cup all-purpose flour and 1 teaspoon cinnamon in a small bowl. Add in 4 Tablespoons salted butter, cut into small cubes.
Using a fork, a pastry cutter, or your fingers, cut the butter into the flour mixture until it reaches a crumbly state.
Sprinkle crumble topping on top of cake batter.
Bake the cake for about 40-45 minutes (until top is golden brown) and an inserted toothpick comes out clean.
Allow to cool for at least 10 minutes before slicing and serving.