Place the 2 ⅓ cups oat flour, ½ cup almond flour, 1 ½ teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon fine sea salt in a medium bowl and whisk to combine.
Beat the 1 cup unsalted butter on medium speed for about 30 seconds, then beat in the 1 ½ cups granulated sugar until fluffy. Scrape down the sides of the bowl and add the 3 large eggs one at a time, then beat in the 1 teaspoon pure vanilla extract.
With the mixer on low speed, beat in half of the flour mixture until just combined, then beat in the ¾ cup full-fat sour cream and the remaining flour mixture until just combined. Fold in the 1 ½ cups fresh or frozen blueberries.
Transfer the batter into the tube pan and smooth the top. Preheat the oven to 350℉.
Bake until a tester inserted into the cake comes out clean or with only a few moist crumbs, the cake bounces back when lightly pressed, and the top is golden-brown, 45 to 50 minutes. Place the Bundt pan on a wire rack and let cool for 30 minutes. Flip the cake out onto the rack and let cool completely, about 1 hour more.
Notes
Room temperature ingredients. For the best results, bring the butter to soft-ish room temperature on the counter before starting to make the cake. This will help it to seamlessly blend into the batter.
Rest the batter. Like all gluten-free batters, make sure to rest the batter before baking to fully hydrate the flours. This will create that lovely, fluffy crumb.
What kind of pan? This is a hefty cake, and I love baking it in a tube pan. However, a large bundt pan (grease it very well) or 2 9-inch cake rounds also work. You can bake it as a layer cake and fill and frost it with a lemon or vanilla buttercream.
To serve: Fresh blueberries, vanilla ice cream, or whipped cream.