Muffin batter. In a large mixing bowl, using your fingertips, press the lemon zest into the sugar. Whisk in the flour, baking powder, salt, and cornstarch.
In a medium mixing bowl, whisk together (use the whisk from the dry ingredients) the sourdough discard, sour cream, eggs, melted butter, vanilla and lemon juice.
With a rubber spatula or spoon, mix the wet ingredients into the dry ingredients, just until there is barely any flour still visible. Do not overmix.
Cover and refrigerate at least a couple of hours or overnight.
Crumbs. Combine all ingredients and mix with a fork, just until crumbs are formed. Refrigerate until ready to use.
Preheat oven to 400° F.
Line 12 muffin tins with papers or grease the tins with butter or non-stick cooking spray. Spread approximately 1 Tablespoon of the cold muffin batter on the bottoms of each paper or muffin tin. Mix the blueberries into the rest of the batter, being careful not to overmix. Fill the muffin tins with equal amounts of the cold batter. Sprinkle the tops with the crumbs.
Bake for 23-27 minutes, just until a toothpick inserted in the middle comes out clean.
Cool in pan on a wire rack.