Preheat oven to 350° F. Butter or lightly grease the bottom of a 9×13 pan.
Place nectarines, blackberries, cinnamon, salt, sugar and water in the pan. Toss gently to coat the berries in the sugar mixture.
In a separate medium bowl, mix together all the ingredients for the crumble until the mixture is moist and crumbly. Sprinkle evenly over the top of fruit mixture.
Bake the crumble for 40 minutes. Remove from oven and allow to cool for 10-15 minutes before serving.
Serve warm with ice cream or a dollop of freshly whipped cream.