Preheat broiler to high. Place chicken on a baking sheet coated with cooking spray. Lightly mist chicken with cooking spray and sprinkle evenly on both sides with paprika, chile powder and salt. Broil 6 to 8 inches from heat until center of chicken is no longer pink, about 6 minutes per side. Transfer chicken to a cutting board and let rest for 3 to 5 minutes before cutting into bite-size chunks.
In a blender, purée onion, yogurt, buttermilk and hot sauce. Divide romaine and red leaf lettuces among 4 plates and top with bell pepper, tomatoes, carrots and chicken, dividing each evenly. Serve yogurt mixture on the side or drizzle evenly over each plate. Sprinkle with blue cheese.