Place one of the pie crusts at the bottom of a pie pan. Slice the other pie crust into vertical strips about 1 inch thick.
In a medium saucepan, add 2 cups of blueberries, sugar, cornstarch, and lemon juice. Stir and bring to a boil. Once boiling, and thickened, remove from heat. Add in butter and stir until melted.
Pour the thickened mixture into a large bowl with the remaining blueberries. Stir well.
Pour into the prepared pie crust.
Top with lattice-top crust, folding one over the other in both directions.
Bake the pie in preheated oven for 30 to 40 minutes.