Center a rack in the oven and preheat to 400 (F). Prepare a 9-inch tart pan with a removable bottom by spraying liberally with cooking spray. Seriously, go all out — this crust has a tendency to stick to the pan. Set aside.
In a medium bowl, whisk together 3/4 cup all-purpose flour, 1/2 cup dark rye flour, 1 teaspoon baking powder, and 1/2 teaspoon kosher salt until well combined. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 6 1/2 tablespoons unsalted butter and 1/2 cup granulated sugar. Beat on medium-high speed for 2 to 3 minutes until the mixture is light and fluffy. Use a rubber spatula to scrape down the sides of the bowl, and turn the mixer back on to its lowest setting and add 1 large egg. Continue beating until the egg is well incorporated into the mixture.
Once the egg is fully incorporated into the butter and sugar, Scrape down the sides of the bowl, and turn the mixer back on to its lowest setting and add the dry ingredients (from the 2nd step) all at once. Continue mixing until the ingredients clump together and create a dough.
Turn the dough out onto the prepared tart pan. Use your hands to press the dough into the tart pan, creating an even layer across the bottom of the tart pan and up the sides of the pan. As you do this, be sure to dip and cover your hands into the 1/2 cup of flour that you reserved earlier — the dough can be quite sticky, and flouring your hands prevents the tart crust from sticking onto your hands as opposed to the pan.
When you've finished preparing the tart crust, place the tart pan on a parchment lined baking sheet and set aside. Make the tart filling by whisking together 5.5 ounces sour cream, 1/3 cup heavy cream, 1 large egg, 3 tablespoons sugar, and 1 teaspoon almond extract until well combined.
Pour the tart filling into the prepared crust. Take 10 ounces fresh blueberries and evenly distribute them over the tart filling.
Bake in the preheated oven until the filling has set and the crust is golden, about 25 minutes. When you take the crust out of the oven, the center of the crust should wobble ever-so-slightly, while the outsides remain set. Cool on a wire rack. Serve warm, or chilled from the refrigerator.