You can use white or regular balsamic vinegar. Make sure to use a mild-flavored olive oil. I'd usually tell you to substitute any mustard you want, but please use Dijon mustard in this. You can use any blue cheese you like, but a sweeter, richer one made from cow's milk, compared to sheep's, is ideal.
You can add fresh, crisp apple or pear slices to this as well.
All dressing ingredients are "to taste" and should be adjusted for personal tastes. A bitter greens salad needs a dressing that is relatively acidic.
Add 2/3 of the dressing amount at first, then toss, and adjust with more if needed. There should be a generous amount of dressing used for this salad, and the dressing recipe may make more than you need.