A true double blind test wouldn't be easy to set up, but as a scientific experiment it really isn't very hard. As far as interpreting the results of an experiment goes, I don't think you can design an experiment that isn't open to misinterpretation. It's a very different statement and attitude to say that a DBT has to be well designed and rigorous than to say DBTs serve no useful purpose.
Why is Double Blind Testing Controversial?
I noticed that the concept of "double blind testing" of cables is a controversial topic. Why? A/B switching seems like the only definitive way of determining how one cable compares to another, or any other component such as speakers, for example. While A/B testing (and particularly double blind testing, where you don't know which cable is A or B) does not show the long term listenability of a cable or other component, it does show the specific and immediate differences between the two. It shows the differences, if at all, how slight they are, how important, etc. It seems obvious that without knowing which cable you are listening to, you eliminate bias and preconceived notions as well. So, why is this a controversial notion?