In a small mixing bowl, whisk together the buttermilk and mayonnaise until smooth. Stir in the blue cheese crumbles and scallions. Season to taste with salt and pepper.
Chill the dressing to allow the flavors to blend. Use within one week.
You DO need real buttermilk, not the dried powder, not the add-vinegar-to-milk stuff. Real cultured buttermilk from the dairy case. It costs about $1.99 at Trader Joe's and you can use it in lots of things. (ALDI, please start carrying it. Please.)
Blue cheese or gorgonzola crumbles will both work in this recipe, but for the freshest and best-tasting cheese, buy it in a block and crumble it yourself.