Combine blackberries, blackberry jam, white sugar, cornstarch, and 1/8 teaspoon kosher salt in a small saucepan.

Heat on medium-high heat, stirring constantly, until it boils. Reduce to medium heat and continue stirring for 7 to 10 minutes.

Once thickened, remove mixture from heat and allow to cool.

Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper and spray with cooking spray.

Combine oats, flour, light brown sugar, almonds, cinnamon, baking soda, and salt in a bowl. Stir well. Drizzle with melted butter and mix until fully incorporated.

Press 2 cups of the crumble mixture into the prepared pan. Flatten and smooth with fingers or a measuring cup.

Pre-bake the crust for 10 minutes, then cool for 5 minutes.

Pour the blackberry filling over the crust, then sprinkle with the remaining crumble mixture.

Bake for 40 minutes, or until the top is golden brown and filling is bubbly.