For the crumb topping: Mix together the flour, butter, demerara sugar, granulated sugar and baking powder until combined. Refrigerate for 15 minutes, then break into small crumbs.
For the muffins: Preheat the oven to 200 degrees F.
Spread the bread cubes on a large baking pan or 2 small baking pans. Bake to dry out the bread cubes, about 15 minutes. Set aside to cool.
Raise the oven temperature to 350 degrees F. Line a 12-cup muffin pan with paper liners and spray the liners with nonstick spray or brush with melted butter.
Whisk together the granulated sugar, vanilla, lemon zest and eggs until pale and thick. Whisk in the milk and butter until combined. Stir in the bread and let the mixture sit until the bread has absorbed most of the liquid, about 5 minutes. Whisk together the flour, baking powder and salt in a small bowl, add it to the bread mixture and mix until just combined. Fold in the blueberries.
Divide the mixture among the 12 cups and top each muffin with some of the crumb topping. Bake until lightly golden brown and just set, about 25 minutes. Let the muffins cool briefly in the pan, then remove and let cool on a baking rack.
For the lemon glaze: Combine the confectioners' sugar and lemon zest in a small bowl. Whisk in the lemon juice until you get the perfect drizzling consistency. Let sit for 5 minutes, then drizzle over the muffins.