For the muffins: Preheat the oven to 325 degrees F. Line a muffin tin with paper liners.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar. Cream together until light and fluffy. Add the eggs, one at a time, scraping the sides and bottom of the bowl after each addition. Add the lemon extract and mix to incorporate.
In a small bowl, combine the flour, baking powder, salt, lemon zest and nutmeg and whisk for 30 seconds to evenly distribute the leavening. With the mixer running on low speed, add half of the flour mixture to the butter mixture, then add the buttermilk and follow with the remaining flour mixture, mixing until just incorporated. Using a large rubber spatula, gently give the batter a few quick strokes to make sure there are no pockets of flour remaining. Fold in the blueberries. Fill each muffin cavity three-quarters full.
For the crumble: In a large bowl, combine the flour, butter, granulated sugar, brown sugar and salt using a wooden spoon or your fingers. Keep working the mixture until the butter is absorbed and the mixture starts to clump.
Sprinkle about 1 tablespoon of the crumble on top of each muffin. Bake until the muffins just spring back when gently poked, 15 to 20 minutes.