Preheat oven to 375 degrees F. Lightly coat cups of a 12-cup muffin tin with cooking oil spray (or use muffin cup liners or silicone cups).

Add lime zest and sugar to a small bowl. With your fingers, rub zest into sugar to infuse sugar with lime flavor. In a large bowl, vigorously stir butter and lime zest sugar with a wooden spoon until the mixture is well combined. Add eggs one at a time, stirring after each until eggs are thoroughly incorporated. Add lemon purée and stir to combine.

Combine baking powder and flour in a small bowl and add flour mixture to sugar-butter mixture, alternating with milk, stirring after each addition until nearly combined.

Fold in blueberries until combined. Scoop ⅓ cup of batter into each muffin cup (or use an ice cream scoop). Sprinkle each muffin with 1 teaspoon of crystallized sugar, if using.

Bake for about 30 minutes, or until muffin edges are browned and a toothpick comes out clean when inserted into muffin. (Start checking the muffins at 25 minutes.)

Cool on a baking rack for 10 minutes. Remove muffins and let cool completely.

Store in an airtight container or freeze for longer storage.