Place a large bowl of ice water by the stove. Bring water to a boil in a large pot. Add beans, and boil until tender-crisp, about 5 minutes (they will turn a bright green). (Boil small beans for 2 minutes, medium beans for 3 minutes, and large beans for 4-5 minutes.)

Drain the beans; transfer to the ice water to cool. Drain; pat dry.

Combine buttermilk, blue cheese, mayonnaise, dill, vinegar, sugar, salt and pepper in a mini food processor. Pulse until almost smooth. (Or, optional to leave the blue cheese chunky.)

Transfer the beans to a large bowl, add onion and the dressing; toss to coat.