Rinse the black soybeans thoroughly and soak them for at least 8 hours.
Boil the soaked black soybeans in boiling water for 20 minutes.
Drain the boiled soybeans and let them cool.
Blend the cooled black soybeans with water until smooth.
Strain through a sieve or cheesecloth to extract the soy milk.
Season with salt to taste.
Serve chilled.