Cook pasta in a pot of salted boiling water until just beyond al dente, about 7 to 9 minutes, and cool under cold running tap water. Drain and reserve.
In a small saucepan, bring 2 cups of water and ½ teaspoon salt to a boil over high heat. Blanch snow peas until just tender but still snappy, about 1½ minutes, and transfer to an ice bath to cool. Drain and slice lengthwise into strips about ¼-inch thick. Reserve.
Season tuna with remaining salt and the pepper. Heat 1 tablespoon oil in a 9-inch skillet or sauté pan over high heat; when it shimmers, sear tuna, about a minute and a half on each side. Drain tuna on a plate lined with paper towel, then cut into pieces about ¾-inch long and ¼-inch wide, and reserve.
Whisk remaining oil with ginger juice, rice wine vinegar, soy sauce, sesame oil and mirin, if using. In a large serving bowl toss pasta with dressing. Add snow peas and tuna. Sprinkle with black sesame seeds and scallions and toss thoroughly to combine. Season with salt and black pepper to taste. Refrigerate covered, or serve immediately.