In a medium bowl, whisk together the mayonnaise, sour cream, buttermilk, white wine vinegar, garlic, onion powder, kosher salt, and black pepper until smooth and creamy, 30–60 seconds.
Add the blue cheese and mash about half into the base with a fork while folding in the rest to keep small crumbles throughout.
Cover and chill until flavors meld and the dressing slightly thickens, 30–60 minutes. Stir before serving and keep refrigerated.