Of course extreme anxiety can cause the blood pressure to rise in certain cases. But doctors are very reluctant to prescribe xanax for which it is intended, ie. anxiety and panic attacks. So, I seriously doubt that a doc would prescribe it for high blood pressure.
Now, I'm speaking generally and it would probably depend on your doctor, how long he has known you, his general beliefs etc. But I doubt if a new doc or OP on a first visit would prescribe you xanax for high blood pressure even though you tell them that it works; they would be more inclined to prescribe you something else and advise you to address the cause of your anxiety and high blood pressure.