Mix until combined. Add butter and continue mixing. Stop the mixer and use your fingers or a fork to break up the butter into smaller pieces. Resume mixing.
Add eggs and continue mixing until a crumbly dough begins to form. Set aside. Add blackberries, lemon sugar, cornstarch and sugar to a large mixing bowl and stir together.
Press half of the dough into the bottom of the backing dish. Pour blackberry mixture over the crust and speed with a rubber or silicone spatula until evenly distributed.