Whisk together black bean and garlic sauce, vinegar, 1 teaspoon cornstarch, and water in a bowl. Pat fish dry with paper towels. Place remaining cornstarch in shallow dish. Coat fish with cornstarch, shaking to remove excess.
Heat oil in large nonstick skillet over medium-high heat until just smoking. Add fish and cook until browned and cooked through, about 3 minutes per side. Transfer to a platter.
Add ginger and scallion whites to empty skillet and cook until fragrant, about 1 minute. Add black bean mixture and simmer until slightly thickened, about 1 minute. Pour sauce over fish and sprinkle with scallion greens and cilantro. Serve.