Lightly oil or spray a 10 inch dutch oven and line with parchment paper.
To make the topping: Mix the sugar, flour and cinnamon in a small bowl.
Cut or rub in the butter or margarine with the side of a fork, two knives or your finger tips until it reaches a crumbly state. Set aside.

To make the batter: Blend the flour, baking powder and salt together in a medium sized mixing bowl.
In a large bowl cream together the sugar, butter or margarine, egg and vanilla.
Alternately add the milk and the flour mixture to the creamed mixture, ending with the flour. Stir only enough to blend.
Fold in the blueberries.
Pour the batter into prepared dutch oven and spread evenly. Batter will be wet.
Sprinkle the topping over the batter and bake at 375 degrees for 30-35 minutes or until a cake tester or knife comes out clean.

Let cool in dutch oven for 10 minutes before inverting on a plate. Serve warm.