Grilling method: Heat an outdoor grill to medium-high, direct heat. Oven method: Preheat oven to 425F.
Slice red bell peppers and red onions, and add to a large bowl. Drain and rinse black beans. Add beans to the bowl. Add olive oil, 1 tablespoon of taco seasoning, salt (to taste), and 1/2 teaspoon of the black pepper. Toss to combine.
Prepare 4 (10-inch long) sheets of aluminum foil. Use heavy-duty foil or double sheets to prevent leaking.
Divide the vegetable mixture among the 4 sheets of foil, leaving a 2-inch border on all sides.
Season chicken breasts with the remaining 1 tablespoon of taco seasoning, salt (to taste), and the remaining 1/2 teaspoon black pepper. Place a breast on each vegetable pile.
Fold the sides of the foil to meet over the center. Crimp the edges together to create completely sealed packets.
Grilling method: Place the packets directly onto the grill grates, cover, and grill until the chicken is cooked through and the vegetables are tender, 20 to 25 minutes. Oven method: Place packets on a rimmed baking sheet and roast for 20 to 25 minutes.
Let cool for 10 minutes before opening the packets and serving. Serve with tortillas and your favorite fixings.
