In a medium bowl, combine brown sugar, white sugar, flour, cinnamon, and salt. Add the cold, cubed butter and use your fingertips to blend until coarse crumbs form. Refrigerate until ready to use.

In a large bowl, sift together the flour, baking powder, and baking soda. Stir in poppy seeds and salt. Set aside.

In another large bowl, rub the lemon zest into the sugar with your fingers until fragrant and lightly tinted.

Whisk the sour cream, sourdough discard, melted butter, lemon juice, eggs, and vanilla into the lemon sugar mixture until smooth.

Add the dry mixture to the wet ingredients and stir gently just until incorporated. Do not overmix. Fold in the blueberries carefully.

Preheat the oven to 425°F. Grease or line 24 muffin cups. Divide the batter evenly among them and sprinkle each with chilled crumble.

Place the pan on the center rack and bake for 14–15 minutes, until golden and a toothpick inserted in the center comes out with a few moist crumbs.

Let the muffins rest in the pan for 10 minutes before transferring to a wire rack to cool completely.

Stir lemon juice into powdered sugar until smooth and pourable. Drizzle over cooled muffins before serving.