For the Blunzen sauerkraut sachet, remove strudel leaves from the refrigerator about 10 minutes before processing. Preheat oven to 170°C. Cut each pastry sheet in half and fold in the middle to form a square. Brush edges with egg white. Place one black pudding slice in the center of each pastry square and top with 1 tablespoon sauerkraut.
Pull up corners of dough and fold into a small bag. Place on a baking sheet lined with baking paper, drizzle them with a little melted butter. Bake in the preheated oven on the lower rack for about 15 minutes and serve the Blunzen sauerkraut hot.