Wash beans. Soak them overnight in water with baking soda.
Next day drain black soy beans, keep soaking water and if necessary add more water to get 700 ml, boil it, add sugar and soy sauce, add black beans. Simmer it on low heat about 12 hours, once per hour stir gently and add water if necessary. If there is foam remove it.
Once they are ready you can store them in jar in fridge and use to make Japanese sweets or eat as a snack.