Preheat oven to 350°F. Brush bread with melted butter. If it's already stale, no need to toast it. If it's fresh, toast it in the oven for 5-10 minutes until it's dry but not browned. Lightly butter your baking dish.
While bread is toasting, put lemon zest and sugar into a small dish. Use fingertips to rub lemon zest into the sugar, breaking it up and releasing its flavor.
Whisk eggs and milk in a large bowl. Add vanilla and salt and combine.
Arrange bread in baking dish(es) and sprinkle blueberries over top, nestling a few in among the bread. Sprinkle half the lemon sugar over the bread.
Pour custard over bread, then top with the remaining lemon sugar. Let mixture soak for 15 minutes at room temperature, or up to overnight in the refrigerator.
Bake for 35 to 45 minutes, or until custard sets and a knife inserted into the center of the pudding and turned slightly causes no liquid custard to spill into the crack.