Lay peppers on their sides on racks of gas burners and turn flames on moderately high. Roast, turning frequently with tongs, until skins are blackened, 5 to 8 minutes. (Or broil on rack of a broiler pan about 4 inches from heat, turning frequently, 15 to 20 minutes.)
Transfer to a bowl, then cover, and let steam 10 minutes. Alternatively, place peppers in resealable plastic bags to steam. Peel peppers and discard stems and seeds. Cut lengthwise into 1-inch-wide strips.
In a bowl, place cleaned peppers. Douse with balsamic vinegar, salt, and pepper, to taste.
Peppers can marinate, covered and chilled, up to 3 days.