To prepare dressing, in a screw-top jar, combine vinegar, oil, sugar, mustard, onion powder, salt, and pepper. Cover and shake well. Chill until serving time.
Before serving, in a large bowl, combine shredded cabbage and blue cheese. Shake dressing; add to cabbage mixture and gently toss to coat.
Variation: For 5 servings: Toss half of the dressing with 2½ cups of the shredded cabbage with carrot (coleslaw mix) and 2 tablespoons crumbled blue cheese. Store the remaining dressing in the refrigerator for up to 1 week.
To make ahead: The dressing may be chilled for up to 1 week. The dressed salad may be covered and chilled for up to 24 hours before serving.