Preheat oven to 400°F. Line a mini muffin tin with muffin liners.
Soak poppyseeds in buttermilk at room temperature for 30 minutes.
Meanwhile, whisk dry ingredients, including lemon zest, together in a bowl. Set aside.
Add room temperature eggs and vanilla to buttermilk and poppyseed mixture. Slowly drizzle in melted butter, whisking well until thoroughly combined.
Pour liquid ingredients into dry ingredients and gently fold together with a spatula just until combined. Do not over mix.
Cover muffin batter and let rest 30-60 minutes.
Meanwhile, rub granulated sugar and lemon zest together in a bowl to use for muffin topping.
Use a 2 teaspoon scoop to portion muffin batter into lined mini muffin tin. Press a single blueberry deep into the center of each muffin, then use a toothpick to push (or twirl) the batter over the blueberries to hide them. Top each muffin with ¼-½ teaspoon lemon sugar.
Bake for 13-15 minutes at 400°F. The muffins will not brown on top, but a toothpick inserted should come out clean (as long as you don't piece a blueberry).
Let cool in muffin tin 5-10 minutes, then remove to a rack to finish cooling completely.