Thickening the natural fruit juices from any berry is tricky, especially when the filling is cooked in the pie. (As opposed to pre-cooking the filling.) Blueberries are particularly pesky, natural pectin-wise, and it seems that some varieties gel better than others.  You want your pie-fruit to be thick enough to sit on a fork, but you don't want a gummy, gelatinous mess that feels weird on your tongue and totally overwhelms the fresh, natural flavors of the berries. You also need that crust to bake up nice and flaky. It's a balancing act.  That said, I am not a fan of cornstarch for thickening pies, and I avoid it whenever possible. Sure, thickening with cornstarch is easy, but it often results in what I can only describe as a gloppy, bouncy filling.  For this pie, I use my pie thickener of choice - instant tapioca, plus just a little flour and butter to seal the deal. This combination gives me the most consistent results with my blueberry pie filling, yielding a thick, juicy filling that highlights (instead of hides) all those delicious fresh berry flavors.  Blueberry pies, in particular, thicken a lot when they cool. Don't be concerned if your filling seems thin when you first take it out of the oven. For best results, allow pies to set up completely before cutting in.