The main difference between Roquefort dressing and blue cheese dressing is in the milk that is used to make the cheese that the dressings are made of.
Blue cheese dressing is dressing made from blue cheese that is made from cows milk.
Roquefort dressing is made from cheese and French Sheep's Milk.
Roquefort dressing and blue cheese dressing have an almost similar taste but in my opinion the Roquefort dressing tastes much better than the blue cheese dressing.
But some people find that they both taste the same.