Preheat oven to 350 degrees Fahrenheit.
Grease a 9x13 inch pan. (I used a glass pan). If you're using a dark coated pan, the bottom may over-brown a bit, so just be aware.
In a large mixing bowl, whisk together the flour, baking powder, salt, granulated sugar and packed brown sugar. Whisk well and set aside.
In another bowl, mix together the sour cream, milk, eggs, vanilla extract and juice and zest from one lemon. Whisk well and set aside.
Make sure your butter is slightly cooler than room temperature. Add the butter (in chunks) slowly to your dry ingredients. (You can turn your stand mixer on low and leave it on while you add the butter chunks a bit at a time, or you can use a hand mixer.)
Once all the butter has been added, turn the mixer up to medium and mix until all the flour is coated with the butter and the mixture is crumbly. It will have a sand-like texture.
Pour in about half of the liquid mixture. Mix on medium until just combined. About 15-20 seconds.
Pour the last of the liquid mixture and mix again until combined. Don't over mix the batter. Mixing it for several minutes is too long. Once all the ingredients are incorporated well, stop mixing.
Use a rubber spatula and scrape the sides of the bowl. Mix again for only about 10 seconds.
Add a couple teaspoons of flour to the frozen blueberries a stir to coat them.
Add the flour coated frozen blueberries to the cake batter and stir in with a wooden spoon or rubber spatula, not with the electric mixer.
Pour the batter into prepared pan. If you use frozen blueberries, the batter will become quite thick and possibly a little stiff. It's okay, just spread it evenly into the pan as best you can.
Sprinkle the crumb mixture evenly over the top of the cake batter.
Bake at 350 degrees for approximately 50-55 minutes. The top will be golden brown and a toothpick inserted into the center will come out either clean or with a few moist crumbs. (I used frozen blueberries. If you use fresh blueberries, you may not need to bake as long. Watch your cake closely after about 40 minutes.)
Set on wire rack to cool for at least 30 minutes, then serve straight from the pan. This cake is best served warm.