Preheat your oven to 425-F and spray a 12-cup muffin pan with nonstick cooking spray, and set it aside.
Make the streusel topping by combining the melted butter, dark brown sugar, white sugar, and cinnamon in a small bowl until well mixed. Add the flour and mix until it is thick and crumbly. Pop it into the fridge to chill while you get the muffins ready.
Cream the butter, brown sugar and white sugar in the bowl of a stand mixer with the paddle attachment or with an electric hand mixer.
Beat in the eggs one at a time until well-mixed, scraping down the sides of the bowl as needed.
Mix in the Greek yogurt and vanilla.
Combine the flour, baking powder, baking soda, and salt in another bowl.
Add half the flour mixture to the egg-butter-sugar mixture and mix until combined, then add the remaining dry ingredients. Scrape down the bowl as needed. Your batter will be quite thick.
Using a spatula, fold in the blueberries and rhubarb.
Divide the batter between the muffin cups, filling them about 3/4 full.
Using your fingers or a fork, break up the streusel topping and, keeping it in pea-sized chunks, add it to the tops of the muffins. Some of the topping pieces will be smaller, and that's fine.
Bake the muffins at 425-F for 5 minutes, then lower the temperature to 350-F and continue cooking for 12-15 minutes or until cooked and golden brown on top. An inserted toothpick should have a few clingy crumbs to it but no raw batter.
Cool for 5 minutes in the muffin tin, then remove and enjoy!