In a small bowl, using a fork, combine the blue cheese, mayonnaise and 1/4 cup buttermilk until thick and lumpy. Stir in the remaining 1/4 cup buttermilk and the sugar. Refrigerate.
Quarter, core and coarsely grate the apples. In a medium bowl, combine the apples with the dressing. Stir in the celery; cover and refrigerate until ready to serve.