In a large mixing bowl, gently toss the blueberries with sugar and lemon zest until well combined and the berries start releasing juice.
Transfer the blueberry mixture into a baking dish, spreading it out evenly. The juices will begin to pool at the bottom.
In a separate bowl, combine the flour, oats (if using), cinnamon, and a pinch of salt if desired. Add the cold butter pieces and use a pastry cutter or fork to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.
Sprinkle the crumble topping evenly over the blueberry filling, covering all the berries. It should look crumbly and slightly uneven for a rustic look.
Place the baking dish in a preheated oven at 375°F (190°C) and bake for about 35 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Remove the cobbler from the oven and let it cool slightly for 10 minutes before serving. The juices will thicken a bit as it rests, and the topping will remain crispy and inviting.