Preheat your oven to 325°F. Grease and line a standard 9-inch x 5-inch loaf pan with unbleached parchment paper and set it aside.
First, make the topping.
If you would like to add the optional crumble topping to the bread, make it first, so it can chill while you make the batter for the bread.
Place all the crumble ingredients in a small bowl and mix to combine fully. Place in the refrigerator to chill while you make the bread.
Make the bread.
In a medium-sized bowl, place the flour, xanthan gum, cornstarch, baking powder, and salt, and whisk to combine well. Set the bowl aside.
In a large bowl, place the butter, sugar, eggs, and vanilla, and beat with a handheld mixer until very well-combined.
Add half of the dry ingredients, and beat until just combined.
Add the sour cream, and beat until just combined, followed by almost all the remaining dry ingredients (reserve about 1 tablespoon in the bowl) and beat until just combined. The mixture will be very thick.
Place the blueberries in the bowl with the remaining tablespoon of dry ingredients and toss them gently to coat.
Add the berries to the batter and, using a spatula, fold the blueberries into the batter until they’re evenly distributed throughout the batter.
Assemble the bread.
Transfer the batter to the prepared loaf pan and smooth the top with a moistened spatula.
Remove the (optional) crumble topping from the refrigerator and break it into large irregular pieces with the tines of a dinner fork.
Place the pieces of crumble on top of the batter in the loaf pan and press to help it adhere well.
Bake the bread.
Place the loaf pan in the center of the preheated oven and bake for about 1 hour, rotating at least once during baking, until the loaf springs back when pressed gently in the center and a toothpick inserted in the center (wherever the loaf splits during baking) comes out with moist crumbs attached (but not batter).
Remove the bread from the oven and allow it to cool in the pan for 10 minutes before transferring to a wire rack to cool completely before slicing and serving.