For the pecan crust: Preheat the oven to 350 degrees F.
Place the pecans in the bowl of a food processor fitted with the blade attachment. Pulse until finely ground. Add the flour, granulated sugar and salt and pulse to combine. Add the butter and process until combined. Transfer the mixture to a 9-by-13-inch rectangular baking dish; press flat. Transfer to the refrigerator to chill until firm, about 20 minutes.
Bake until golden brown, 10 to 12 minutes. Remove to a rack to cool.
For the cheesecake filling: Meanwhile, place the confectioners' sugar and gelatin in a medium bowl. Add 1/4 cup cold water and stir to combine. Bring the heavy cream to just a simmer in a small pot over medium-high heat. Add the hot cream to the gelatin mixture and stir to combine and dissolve. Set aside.
Wipe out the food processor bowl to clean away most of the crumbs. Beat the cream cheese and vanilla in the bowl of the food processor until smooth. Add the heavy cream-gelatin mixture and process until smooth, scraping the sides of the bowl as needed. Pour into the cooled nut crust. Refrigerate until firm, at least 1 hour.
For the blueberry topping: Meanwhile, combine the blueberries, granulated sugar and cornstarch in a small saucepan. Stir to combine so that the blueberries are coated with the dry ingredients. Add the lemon juice and stir to combine. Bring to a simmer over medium heat and cook, stirring, until the cornstarch and sugar have melted, about 2 minutes. Set aside to cool at least 30 minutes before pouring over the chilled cheesecake filling. Chill until the topping is set, about 15 minutes, before serving.