Preheat oven to 400 degrees F.
In a medium bowl, stir together vanilla pudding with cornstarch. Set aside.
In another bowl, combine blueberry pie filling with lemon zest.
Fit puff pastry into a 9" tart pan (it does not need to have a removable bottom), until it covers it evenly. You will need to trim the ends, but don't throw them away. Instead cut out little patterns, such as hearts, to decorate your tart. Or fill them with Nutella or extra pie filling and turn them into turnover pockets you can also bake in the oven.
Pour pudding mixture into the tart form.
Add spoonfuls of the pie filling (see picture in the recipe post) and stir carefully with a fork to get a marble effect.
Place tart pan in the middle rack of your oven and bake for 30-40 minutes until the pastry is golden brown and the filling is moving only very slightly. If the pastry is getting too dark, place a piece of foil over the pan.
The filling will still be wobbly, but once it cools down, it will be firm.
Let it cool completely before slicing. Keeps in the refrigerator for up to two days.