Preheat oven to moderately hot (approximately 375°F or 190°C).

Grease a 12-hole muffin pan thoroughly.

In a large bowl, combine the self-raising flour, frozen blackberries, grated apple, and brown sugar.

In a separate bowl, lightly beat the eggs, then whisk in the vegetable oil and buttermilk.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.

Spoon the muffin batter into the prepared muffin pan, filling each hole about 2/3 full.

Prepare the streusel topping: Combine flour, sugar, and mixed spice in a small bowl.

Rub the butter into the dry ingredients until the mixture resembles coarse crumbs.

Roll the streusel mixture into a ball, wrap in plastic wrap, and freeze until firm enough to grate (about 15 minutes).

Coarsely grate the frozen streusel topping over the muffins.

Bake in the preheated moderately hot oven for approximately 20 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the muffins cool in the pan for a few minutes before transferring them to a wire rack to cool completely.