Preheat your oven to 375°F. Line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate bowl, mix together the coconut milk, melted coconut oil, and vanilla extract. Make sure your coconut milk is room-temperature so your melted coconut oil doesn't solidify.
Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the blackberries.
In a small bowl, combine the streusel topping ingredients: flour, brown sugar, vegetable shortening and ground cinnamon. Use your fingers or a fork to mix until it resembles coarse crumbs.
Spoon the muffin batter into the prepared muffin tin, filling each cup about 3/4 full.
Sprinkle the streusel topping evenly over the muffin batter in each cup.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5-10 minutes, then transfer them to a wire rack to cool completely.