In a large bowl, whisk the starter, sugar, egg, warm milk, and melted butter until smooth. Add flour and salt, mix into a shaggy dough, cover, and rest 30 minutes.

Perform a set of stretch-and-folds: pull one edge up and fold over the center, rotate, repeat for four folds. Cover and repeat two more sets 30 minutes apart, for a total of three sets over 90 minutes.

Beat cream cheese, 2 tbsp sugar, vanilla, and lemon zest until smooth. If using frozen blueberries, toss with a bit of flour.

Butter a 9x5-inch loaf pan. Roll dough into a 12x8-inch rectangle. Spread cream cheese mixture leaving a border, scatter blueberries, roll up tightly, seal, and place seam-side down in pan.

Cover and let rise in a warm spot until puffy and just above the rim, 2 to 4 hours depending on temperature. Dough should pass the poke test.

Preheat oven to 375°F (190°C). Bake for 45 minutes until golden and internal temp reads 200–205°F (93–96°C). Cool in pan 10 minutes, then turn out onto a rack. Slice and serve warm.