In a blender mix the blackberries, sugar, and lemon juice until a smooth puree. Next, strain the puree through a fine mesh sieve to remove all the seeds. Use a spatula to fully strain out the seeds. Transfer 1/4 cup to a small bowl, refrigerate and save for topping. 

Combine the cream and vanilla. Whip with a hand or stand mixer until stiff peaks form (DO NOT OVERMIX). Using your spatula, fold in the blackberry puree (be gentle and careful not to over mix). Cover and refrigerator for at least an hour before serving. 

Crush graham crackers and mix with melted butter to form the crumble. Put the crumble mixture in the refrigerator as well. 

To serve, put a small amount of crumble at bottom of bowl, add a decent spoonful of mousse, drizzle the leftover blackberry puree on top, and throw some more crumble on top.