Preheat the oven to 375 degrees F. Lightly butter an 8-inch square baking dish.
For the filling: In a bowl, add the blueberries, sugar, cornstarch, cider, and vanilla, Carefully mix the ingredients together with a spatula, leaving the blueberries whole. Set aside while you assemble the topping.
For the topping: In the bowl of a mixer, add the flour, sugars, and butter. Using the paddle attachment, incorporate the butter into the dry ingredients until the mixture looks mealy or crumbly. (This can be done by hand.) Stir in the oats and nuts.
Pour the filling into an 8-inch square baking dish and evenly distribute the topping over the fruit. Bake for 30 minutes, or until the topping is golden brown and the fruit is bubbling. Cool slightly and serve warm.