Fill a large saucepan with 1 inch of water and fit with a steamer basket. Bring the water to a boil. Place the broccoli in the basket, cover, and steam until tender, 5 to 7 minutes. Rinse with cold water to cool.
Meanwhile, in a small bowl, stir together the buttermilk, sour cream, blue cheese, scallion, vinegar, ½ teaspoon salt, and ¼ teaspoon pepper. Drizzle the broccoli with the dressing and sprinkle with the walnuts.