Wash black soybeans thoroughly and soak them in water for at least 6 hours.
Boil the soaked black soybeans in boiling water for 10 minutes.
Blend the cooked black soybeans with water and salt until smooth.
Strain the mixture through a sieve or cheesecloth to separate the liquid.
Transfer the finished black soybean soy milk to a pot and gently heat over low heat for 5 minutes, stirring constantly.