Add the sugar to a medium sized mixing bowl. Zest the lemons into the sugar, then use your fingers to rub the lemon zest into the sugar.
Melt the butter, then mix it into the sugar mixture.
Add in the kefir, egg, vanilla, and lemon extract. Whisk until well combined.
Add in the flour, poppy seeds, baking powder, and salt. Fold the dry ingredients into the mixture until only a few streaks of flour remain.
Add the blueberries to a separate small bowl, and toss them with 2 tsp. of flour to coat them.
Add the blueberries into the batter, and gently fold them into the batter just until no streaks of flour remain.
(OPTIONAL) Cover the bowl with plastic wrap and let the batter rest at room temperature for at least 30 minutes and up to 1 hour before baking. This will encourage a more flavorful, higher-rising muffin!
Evenly distribute the muffin batter between the six parchment liners, then top the muffins with the lemon 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 10-12 minutes, or until a toothpick inserted into the center of the muffins comes out clean.
Serve and enjoy!