For the cake: Preheat the oven to 375 degrees F.
Spray a 9 by 9-inch glass baking dish with nonstick spray and set aside.
In a medium mixing bowl whisk together the flour, baking powder, salt and ground ginger. Set aside.
In the bowl of a stand mixer, with the paddle attachment, beat together the butter and sugar on medium speed until light and fluffy, approximately 1 minute. Add the egg and beat until well incorporated, approximately 30 seconds. Add 1/3 of the flour mixture and beat on low speed just until incorporated and then add 1/3 of the milk and beat until incorporated. Repeat, alternating flour and milk until everything has combined. Gently stir in the blueberries and pour the mixture into the prepared baking dish.
For the topping: In a small bowl combine the sugar, flour and nutmeg. Add the butter and work into the dry ingredients using a fork to combine. Continue until the mixture has a crumb-like texture. Sprinkle the mixture on top of the cake. Bake on the middle rack of the oven for 35 minutes or until golden in color. Cool for at least 10 minutes before serving.