Whisk together sour cream, mayo, vinegar and buttermilk in a small bowl. Stir in bbq sauce. Season to taste with salt and pepper. Refrigerate until ready to use.
Preheat grill to medium high heat. Liberally season steak with spice rub. Grill to desired doneness. Set aside to rest.
Lower heat to medium. Lightly spray romaine hearts with olive oil. Season with salt and pepper. Grill, cut sides down, for 2-3 minutes.
Cut steak into thin slices. Place one Romaine heart half on a plate along with ¼ of the steak, tomatoes and blue cheese. Drizzle with buttermilk-BBQ dressing. Repeat with remaining Romaine hearts. Serve immediately.