Blue cheese dressing has carb in it: around 1 carb/tblspoon. So be careful not to use too much. (On the label usually there is 0 carb, as they can claim it is zero, if ONE protion has less than 1 carb. One protion is 1 tblspoon or less.)
Stay away from fat free dressings.
For info go to fitday.com. It is FREE and you can find - objective - carb counts there.