Mash the blackberries and set them aside.
Add the sugar to a medium sized mixing bowl, then zest the orange into the sugar. Use your fingers to rub the zest into the sugar.
Melt the butter, then mix it into the sugar. Then, whisk in the egg, vanilla, and greek yogurt.
Add in the flour, baking powder, baking soda, and salt. Use a rubber spatula to fold the mixture together just until the last streak of flour dissapears. If the batter is a little bit lumpy that's okay.
Add in the mashed blackberries, and fold the mixture a few times just to get some swirls of blackbery throughout the batter.
Now it's time to layer the muffins. Start by pouring ~2 tbsp. of muffin batter into the bottom of each muffin liner, then top each muffin with 1 tbsp. of streusel.
Distribute the remaining muffin batter evenly (~3 tbsp. / muffin), followed by the remaining streusel.
Bake the muffins at 425°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F / 175°C.
Bake for an additional 10-15 minutes, or until a toothpick inserted into the center of the muffins comes out clean.
Serve and enjoy!