Bring a large pot of salted water to a boil. Fill a large bowl with ice water and add salt to it until it tastes lightly salty. When the water comes to a boil, add the beans and cook until just tender, about 3 minutes. Drain, then refresh in the water bath to keep the bright green color, and drain well again. Put the beans in a bowl.
In a blender, combine the creme fraiche, lemon juice, olive oil and salt and pepper to taste. Blend until smooth. Pour over the green beans. Add the olives and toss. Garnish with the dill sprigs.