Preheat oven to 400 degrees. Grease a muffin tin or line with muffin liners.
Add flour, sugar, salt and baking powder to a large bowl. Whisk to combine.
Add oil to a 1 cup measuring cup. Add egg and enough milk (⅓ - ½ cup) to fill to 1-cup line. Add vanilla and almond extract (if using) and whisk to combine.
Add to flour mixture and stir until just combined. Then fold in blackberries.
To Make Streusel Topping: Mix together sugar, flour, butter, cinnamon and cardamom with a fork until coarse crumbs form. Sprinkle over muffins before baking.
Divide batter evenly into muffin cups. Sprinkle with streusel topping.
Bake for 20-25 minutes, until a tester inserted into the center comes out clean. When muffins are done, cool for a few minutes in the muffin pan before removing to cool on a wire rack.
Notes
To store, place in a plastic bag or airtight container lined with a paper towel (this prevents them from getting soggy). Seal and store at room temperature for up to 4 days. Or freeze for up to 3 months.