Trim stem ends from beans, remove any strings, and cut beans into 3-inch lengths; rinse.
Set a wok or 5- to 6-quart pan over high heat. When pan is hot, add 1 cup water and beans. Cover and stir occasionally until beans are tender to bite, 4 to 5 minutes. Drain beans and pour into a bowl.
Meanwhile, in a small bowl, mix black bean sauce, 3/4 cup water, and cornstarch.
Return pan to high heat and add oil. When hot, add pork and stir-fry until meat is crumbled and lightly browned, about 3 minutes.
Add onion and stir-fry 1 minute. Add black bean sauce mixture and beans. Stir until sauce boils, about 1 minute.