Toss together the coleslaw mix and onion in a bowl. In a saucepan, mix the vinegar, sugar, and 1 teaspoon salt. Bring to a boil. When sugar has dissolved, pour over the slaw mix and onion. Toss to coat, and let stand 15 minutes.
Drain the slaw mix in a colander, and return to the bowl. Toss with mayonnaise, sour cream, and blue cheese. Season with salt and pepper. Cover and chill at least 1 hour in the refrigerator before serving.