Pat the crabs dry and dredge lightly in flour. Shake off excess. In a large skillet, melt 2 tablespoons of the butter over medium-high heat. Place 4 crabs in skillet, back-side down, and cook until lightly browned, about 2 minutes. Turn and cook until brown on the other side. Remove to a heated platter and keep warm in a low oven. Cook remaining crabs in remaining 2 tablespoons butter in the same way.
Increase heat to high. Pour the sake into the skillet and bring to boil, using a wooden spoon to scrape any browned bits out of the pan. Add the green onions and garlic and boil 1 minute until onions are limp. Add the black beans and boil 1 minute longer, stirring constantly. Spoon sauce over crabs and serve. Serves 4.
» Sake is a sweet Japanese wine frequently served heated with meals. It is available in most liquor stores. If unavailable, substitute white wine, vermouth or dry sherry.