Preheat oven to 175 C / 350 F. Lightly oil and line a 20 cm / 8 inch square baking pan with baking paper.
In a large bowl whisk together the dry ingredients. In a medium bowl whisk the almond milk and vinegar together until frothy, then whisk in the rest of the wet ingredients except for the blackberries. Pour the wet mix into the dry mix and stir until just combined, fold through the blackberries. Scrape the batter into the prepared pan.
To make the streusel, place all of the ingredients except the oil into a small bowl and mix together well. Drizzle the oil over the mixture 1 tbsp at a time, mixing it in with your fingers after each addition. Add enough oil for large crumbs to form. Scatter the crumbs evenly over the batter.
Bake for 35-40 minutes until the topping is toasty and a toothpick inserted into the centre of the cake comes out clean (if there are a few cooked crumbs attached this is okay). Allow the cake to cool in the pan for 30 minutes, then remove to a cooling rack to cool completely, or tuck in while it's still a little warm.