Lightly roll out each puff pastry sheet to a 9×12-inch rectangle on a floured surface. Cut each sheet into 12 3×3-inch squares. Grease two 12-hole mini muffin pan cups. Place one pastry square into each cup. Prick the bottoms several times with a fork. Bake for 20 minutes or until golden brown and pastry is cooked completely. Remove from pans while warm. Transfer to a cooling rack. Cool before filling.
Orange Custard: In a medium, heavy saucepan, combine the milk, sugar, orange zest and juice. Bring to a gentle boil over medium heat, whisking to dissolve the sugar. Remove from the heat. In a medium bowl, whisk together the egg yolks and cornstarch until pale yellow, about 2 minutes. Slowly add 1/2 cup of the hot milk mixture to the egg yolks, whisking constantly until smooth. Add the egg yolk mixture to the pan of hot milk and whisk to combine. Return to medium heat and cook, stirring constantly with a metal spoon until the mixture thickens enough to coat the back of the spoon, about 3 minutes. Remove from the heat. Add salt and vanilla. Chill well before using.
To assemble; arrange tart shells on a tray, spoon in custard. Top with fresh blueberries. Heat marmalade in a small saucepan. Push marmalade through a strainer into a bowl. Lightly paint blueberries tops with marmalade. Top with orange zest. Serve chilled. Best when served within 12 hours.