Preheat an outdoor grill for high heat and lightly oil the grate. Generously grease a sheet of aluminum foil with oil.
Bring barley and water to a boil in a saucepan. Cover, reduce heat to low, and simmer until the barley is tender, 10 to 15 minutes.
Mash black beans with a fork in a large bowl until thick and pasty. Stir barley, Cheddar cheese, mushrooms, onion, red bell pepper, parsley, eggs, garlic, salt, and black pepper into mashed black beans. Mix enough bread crumbs into bean mixture to form a sticky batter that holds together.
Divide batter into 4 to 6 patties; place on prepared aluminum foil.
Grill patties on the aluminum foil for about 8 minutes on each side.