Combine the first 3 ingredients. Spread the mayonnaise mixture evenly over 4 bread slices. Top each with 2 tomato slices and 2 tablespoons cheese; sprinkle with salt. Spread each remaining bread slice with 2 tablespoons black bean dip; place, dip sides down, on top of sandwiches. Heat a large nonstick skillet coated with cooking spray over medium heat. Coat each side of sandwiches with cooking spray. Place sandwiches in pan; cook for 2 minutes on each side or until golden brown.