The flight test of a new aircraft involves a great deal of instrumentation and real-time monitoring by excellent pilots and engineers. Mysteries to field pilots, like Blade Stall, are easily measured and diagnosed as they start to occur in test aircraft, and watching these conditions develop is the craft of a modern test team. On all the test aircraft I flew, we had cockpit indications of the degree of stall, giving plenty of warning prior to any large stall event.
It is highly unlikely that the cause will be determined by a group of guys throwing darts at this forum, much more likely that a tragic set of individually obscure circumstances produce an unlikely chain of events that led to the mishap.
The craft of development helicopter flying is similar to clearing a mine field, where the object is to find the hazards before they trigger massive problems, using tools that find the problems before they become catastrophic. It takes about 1600 flight test hours to certify a helicopter type, and every one of those hours has a finite probability of catastrophe, even the mundane maintenance test flights and avionics check flights.
I know the Bell team well, they are pros, as good as anyone at this game. This has probably shaken them up because we all believe with confidence that our systems work well, and that data, careful test procedures and training will win every time. None the less, the nature of the unknown is to be unknown, and every now and then one of those mines goes off.
This accident is probably not an measure of the professionalism of the team, or of the innate airworthiness of the aircraft, it is a measure of the uncertainty of exploration, and the cost of being surprised.
A quote that helps explain it comes from racing driver Jackie Stewart, "Was I ever scared while driving? If I wasn't scared, I wasn't driving fast enough."