The reason why blood sausage needs to be cooked for about 15-20 minutes is that blood sausage is mainly made from pig's blood and pig's intestines, of which pig's blood is a liquid and usually needs to be boiled to be cooked. Pork intestines, on the other hand, need to be boiled for some time to soften them and to allow impurities and off-flavors to be removed from the intestinal walls. Therefore, you need to cook the blood sausage for about 15-20 minutes to ensure that both the pig blood and the pork sausage are cooked to perfection, thus making the blood sausage more flavorful.