Melt unsalted butter for the crumble. In a bowl, mix melted butter, 3 tbsp sugar, 1/3 cup flour, and 1/8 tsp salt. Refrigerate for 15 minutes.
Grate 1 tablespoon lemon zest using a Microplane or fine grater. Rub zest into the remaining 1 cup sugar (plus butter if using room temperature).
Preheat oven to 350°F. Line a loaf pan with parchment paper.
Attach a stand mixer’s paddle attachment. Add the zest-infused 1 cup sugar and remaining butter (if refrigerated from earlier). Mix on low until aromatic.
Gradually add 1 1/2 cups flour, 1 tsp baking powder, and 1/2 tsp salt into the mixer. Mix for 3 minutes on low speed.
In a separate bowl, whisk eggs and milk until smooth. Pour into mixer and blend until fully incorporated.
Fold in 1 1/2 cups blueberries (coat frozen berries with 1 tsp mint oil if using them).
Sprinkle half the crumble topping over the batter in the loaf pan, gently pressing it into the mix.
Top with remaining crumble and additional blueberries. Bake for 50-60 minutes, testing doneness by pressing gently and ensuring springback.