Preheat the oven to 350°F and line an 8x8-inch metal pan with parchment paper.
In a large bowl, stir together the melted butter, flour, oats, brown sugar, cinnamon and salt. The mixture will be dry and sandy with some larger, well-formed crumble pieces. This makes up the crust and crumble. Set 1 heaping cup (I used almost 1 1/2 cups) of the mixture aside for the crumb topping.
1 cup (2 sticks; 227g) unsalted butter, 2 cups (250g) all-purpose flour, 1 1/2 cups (150g) old-fashioned rolled oats, 1/2 teaspoon salt, 1 cup (200g) light brown sugar, 1 teaspoons cinnamon
Transfer the remaining crumb mixture to the prepared pan, and use a spatula or your fingers to press the mixture evenly into the bottom of the pan to create an even crust.
In the same or a separate large mixing bowl, add all ingredients for the blueberry layer and toss to coat the berries in the sugar and cornstarch. Dump the blueberries over the bottom layer.
16 ounces fresh blueberries, 1/4 cup granulated sugar, 2 Tablespoons fresh lemon juice, 1 Tablespoon cornstarch
Sprinkle the reserved crumb topping over the blueberries. There will be blueberries still showing, this is what you want.
Bake in the preheated oven for 50-55 minutes, or until the crumble is golden and the blueberrries are bubbling around the edges. Check the bars with 15 minutes remaining and cover with foil if the top is starting to look to brown.
Allow the bars to cool for 30-60 minutes then use the parchment paper overhang to remove the bars and cool them completely. You can place them in the freezer to speed up the cooling process. Slice cooled bars into squares and enjoy.