Place onion in a small heat proof bowl. Bring vinegar, salt, sugar and 1/4 cup of water to a boil, pour over onion and stir. Cover and let pickle in refrigerator for an hour.
Whisk buttermilk, sour cream and chives together until smooth add blue cheese.
Drain onions and add to dip. Season with salt and pepper. Can be made 3 days ahead.
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