Broil peppers 4 to 5 inches below heat source, turning every 5 minutes, until skins are blackened, about 25 minutes. Transfer to a large bowl, cover with a large plate or plastic wrap and let steam until cool enough to handle. Peel peppers, trim, discard seeds and ribs, and cut lengthwise into 1/2-inch-wide strips.
In a serving bowl, toss together peppers, oil, vinegar and garlic; season with salt and pepper. Let stand at least 1 hour before serving. (Salad can be refrigerated, tightly wrapped, for up to 3 days.)
Just before serving, toss olives with peppers. Sprinkle salad with mint and serve at room temperature.