Preheat oven to 350 degrees. Line a square 9x9 baking pan with foil, leaving enough overhang to pull out later. A 9x13 pan can be used for a thinner bar but your baking times will vary, the square pan makes quite thick bars.
In a medium bowl combine the melted butter, sugar, vanilla, salt, and 1/2 lemon zest until well combined. Stir in the flour until combined. Reserve about 1/2 cup of the crust to use as crumble on the top. Add oats to the reserved 1/2 cup for the topping. Set aside.
Press remaining dough evenly into the prepared baking pan. Bake for 18-20 minutes.
While the crust is baking, prepare the filling. In a stand mixer or large mixing bowl, beat cream cheese until smooth. Beat in eggs. Add sugar, yogurt, other 1/2 lemon zest, lemon juice, lemon extract, and salt. Scrape sides as needed. Add flour and mix just until combined. Carefully fold in the blueberries.
Pour filling into baked crust. Crumble the reserved crust dough over the top.
Bake for 55-60 minutes. Allow to cool completely before refrigerating. Chill for at least 1 hour before slicing. Lift bars out of the pan with the foil handles, and slice. Store covered in the fridge.
These bars are thick. Thick crust an filling. If you want a thinner bar, try making in a 9x13 pan. Your crust baking time and overall filling baking time will be reduced. Try cutting the times in half, check for done-ness.
A round pie plate would also work well for these bars, and would make really pretty wedges for serving.
I've only made these with fresh blueberries but frozen would work just fine, and you should not need to thaw them.