In a large bowl whisk together mustard, vinegar, shallot, and 1/2 tablespoon oil to make dressing.
In a small heavy skillet heat remaining 1/2 tablespoon oil over moderately high heat until hot but not smoking and saute pecans with salt to taste, stirring frequently, until a shade darker, about 1 minute.
Transfer nuts to paper towels to drain and cool.
Coarsely chop nuts.
Have ready a bowl of ice and cold water.
In a large saucepan of boiling salted water blanch beans until just tender, about 3 minutes, and drain in a colander.
Transfer beans to ice water, stirring until just cool.
Drain beans well and add to dressing.
Crumble blue cheese over beans and gently toss with half of nuts and salt and pepper to taste.
Serve beans at room temperature topped with remaining nuts.