Start off making the pie dough that you will use for your crust. You will get two crusts out of this, but will only need one.
For the Crust
Whisk dry ingredients together.
Cut in butter in until the largest butter pieces are pea sized. You can use a pastry cutter or food processor. Make sure the butter is very chilled, you can dice it up and put it in the freezer for 10 minutes to make sure it's cold enough.
Mix the sour cream and 1/2 cup of the water and mix into the crust. You do not want the crust to be too wet.
Add water until the crust just barely comes together. This is easier to see if you are using a food processor, but doing it by hand is totally fine as well. Don't overwork the crust.
Divide in half and place in fridge while you make the filling.
For the Filling
In a medium saucepan, combine one of the pints of blueberries and the sugar.
Moderately mash the blueberries with a potato masher or a spoon and stir to combine with the sugar.
Let this mixture come to a simmer, slightly mashing and stirring along the way. Simmer until the mixture is reduced by half and thick, about 10 minutes.
Combine the thickened blueberry mixture, the additional pint of blueberries, peaches, cornstarch, and lemon zest and stir until combined.
Spray an 8x8 baking with cooking spray. Pour filling mixture into the dish and make sure it's spread evenly using a rubber spatula.
Remove one pie crust from fridge and pat lightly into a square. Roll it out into an 8x8 square, rotating the dough 90 degrees between rolls as you go. This will help prevent sticking and help maintain the square shape.
Place square on top of filling, tucking under any edges that don't quite fit. If you have a lot of excess, trim it off with a sharp knife or kitchen shears.
Brush the the top with the egg was and cut four vent slits in any pattern in the top of the dough.
Sprinkle with sugar.
Bake at 375 degrees F for about 45 minutes, or until the crust is dark golden brown.