The idea behind blow dry bars is that it's quick and cheap, and caters for huge volumes of people. There is one in Selfridges.
The difficulty will always be recruitment, as good hairdressers wouldn't want to just do blow dries, and there is little money unless you do tons. So it is hard physical work for a low return.
If you are in an area full of professional women who need to be groomed, and can find staff who will work early /late , and can get the huge volumes in, then it would work. In a small town/provinces, I can't see it.
The London ones are still going, but I have no idea how they are getting on.