In a large bowl, combine 2 cups mayonnaise, ¼ cup finely chopped onion, ¼ cup finely chopped green olives, ½ cup finely crumbled blue cheese, and ½ teaspoon garlic powder.
Whisk in ¼-½ cup buttermilk until the dip reaches your desired consistency. Cover and refrigerate for at least 2 hours. Stir before serving, adding a splash of buttermilk if needed.
Notes
Storing and Make Ahead
In the Refrigerator: Store in an airtight container in the fridge for up to 3 days. Stir before serving.
Make Ahead: Make up to 24 hours in advance; the flavors develop even more when the dip sits overnight.