Rinse dried black beans thoroughly and soak them in plenty of water overnight (8-12 hours). Drain the soaked beans and place them in a pot with 3 cups of fresh water. Bring to a boil, then reduce heat and simmer for 45-60 minutes, or until the beans are tender but still hold their shape. Drain the cooked beans and let them cool completely to room temperature. In a bowl, combine the cooled black beans, miso paste, and sea salt. Mix well, ensuring the miso evenly coats the beans. Pack the mixture tightly into a clean, sterilized glass jar, pressing down to remove any air bubbles. Leave about 1 inch of headspace. Loosely cover the jar with a lid, or use an airlock if you have one, to allow gases to escape during fermentation. Place the jar in a cool, dark place at room temperature (ideally 70-75°F / 21-24°C) for 1 to 3 weeks. Check periodically; you may see some bubbling or a slight change in aroma. Taste a bean after 7-10 days to check for desired fermentation and flavor. Once fermented to your liking, tighten the lid and transfer the jar to the refrigerator. The fermented black beans will keep for several months.