1/2 cup mayonnaise
1/2 cup sour cream
1/4 cup plus 2 tbsp. well-shaken buttermilk
1/2 cup crumbled sharp blue cheese, such as Pt. Reyes Blue
1 to 1 1/2 tsp. lemon juice
1 to 1 1/2 tsp. Champagne vinegar
Kosher salt and freshly ground pepper
Put mayonnaise, sour cream, buttermilk, blue cheese, lemon juice, and vinegar in a blender and blend until smooth. Season to taste with salt and pepper.
Make ahead: Up to 4 days, chilled.