If the goal were only to conduct a test that was difficult to pass—like the qualifying exams for some of the more esoteric and restrictive high-IQ societies—then trying to explain principles, scenario design, and how models work would be inappropriate.
If the measure of success for the Fed in administering a stress test was simply how many banks failed, then greater transparency would indeed be a mistake. But that is not the purpose of stress testing, and it never has been.