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 perfect for salads, stir in a tablespoon or two of milk, buttermilk, or water. Taste and adjust with additional lemon juice, salt, and pepper.
Keep, tightly covered, in the refrigerator for one week.
