Beat the balsamic vinegar in a bowl with the sugar, garlic, salt, and pepper until sugar and salt dissolve.
Then beat in the oil by droplets, whisking constantly. (Or place all the ingredients in a screw-top jar and shake to combine.)
Taste and adjust the seasonings.
Toss a few tablespoons of the dressing with the salad mix and desired salad ingredients, top with blue cheese and serve immediately.
If not using dressing right away, cover and refrigerate, whisking or shaking again before use.