Rinse fresh blue mussels in tap water and set aside. Discard any mussel that does not close after being rinsed.
Combine water, soy sauce, Sherry, maple syrup, vinegar and corn starch in a small bowl. Whisk well and set aside.
Heat vegetable oil in a large, heavy bottomed pot over medium heat. Add garlic and ginger and stir fry, stirring constantly, for 1 minute. Add black beans and soy mixture. Stir and let simmer for 1 minute. Add mussels to the pan, cover and leave to simmer for 5-7 minutes, until mussels are open. Discard any mussels that do not open. Toss mussels in sauce and divide between 4 bowls. Garnish with chopped cilantro.