In a medium bowl, whisk together the mayonnaise, sour cream, and milk until well combined.
Add the crumbled blue cheese to the bowl and use a fork or a whisk to mash it into the dressing until it's mostly smooth with a few chunks of cheese remaining.
Add the lemon juice, garlic powder, salt, and black pepper to the bowl, and whisk everything together until the ingredients are evenly distributed.
Taste the dressing and adjust the seasoning if necessary. If the dressing is too thick, add a little more milk to thin it out.
Transfer the dressing to a jar or airtight container and refrigerate for at least 30 minutes to allow the flavors to meld together.