Heat broiler with rack set 4 inches from heat. In a small saucepan, combine blueberries, grapes, ginger, shallot, and 1/2 cup water; season with salt and pepper. Bring to a boil over high heat. Reduce heat to medium-high; cook, stirring occasionally, until grapes break down and mixture thickens, 20 to 25 minutes. Stir in 1 teaspoon vinegar.
While the chutney cooks, place tenderloin on an aluminum foil-lined baking sheet. Rub with 2 teaspoons mustard; season with salt and pepper. Broil, turning occasionally, until an instant-read thermometer inserted in the thickest part of meat registers 145 degrees, 15 to 20 minutes. Cover with foil and let rest 10 minutes before thinly slicing and serving.
Meanwhile, in a large bowl, whisk together oil, remain-ing tablespoon vinegar, and remaining teaspoon mustard; season with salt and pepper. Just before serving, add watercress and toss. Serve pork with chutney and salad.