In a 10-inch skillet, heat oil over medium heat. Add sweet potato, onion, and sweet pepper. Cook, covered, 5 to 7 minutes until sweet potatoes are nearly tender, stirring occasionally. Add black beans and salsa. Cook, uncovered, 5 minutes more or until vegetables are tender and sauce thickens slightly. Season with salt and black pepper. Spoon filling into tortillas. Top with queso fresco and cilantro. Serve with lime wedges, if desired.