Combine flour, sugar, cinnamon, salt, pecans, and butter in a bowl of an electric mixer fitted with a paddle attachment. Mix on low speed until the butter is the size of peas. Set aside.
In a large bowl, combine blackberries, sugar, flour, and vanilla. Pour blackberry mixture into a casserole.
Crumble the topping mixture evenly over the blackberries. Bake for 45 to 50 minutes or until the tops are browned and crisp and the juices are bubbly. Serve warm with Goat's Milk Ice Cream (page 184 of Tracking the Outdoors In) or basic whipped cream.