Steamed Blue Crabs With Black Ginger Dipping Sauce

Ingredients

• 1/2 cup fresh cilantro stems, coarsely chopped
• 36 live blue crabs (1/4 to 1/3 lb each; 3 dozen), well rinsed
• 1 1/2 to 2 tablespoons finely chopped peeled fresh ginger
• 1/2 cup fermented fine-quality soy sauce
• 1/4 cup sugar
• 1/4 cup water
• 3 tablespoons Chinese black vinegar* (preferably Chinkiang) or cider vinegar
• 1/4 teaspoon salt
• Special equipment: a wide 6- to 8-qt heavy pot with a tight-fitting lid