Combine the sour cream, mayonnaise, blue cheese, parsley, lemon juice, salt, and pepper in a small bowl and stir until well blended and creamy. Use the back of a spoon to press some of the blue cheese crumbles into the sauce for even more flavor. The dressing will be thick, which makes it perfect for dips. For a pourable dressing that’s perfect for salads, stir in a tablespoon or two of milk, buttermilk or water. Taste then adjust with additional lemon juice, salt, and pepper. Keep, tightly covered, in the refrigerator for one week.