Re:
I probably didn't express myself clearly,
There is positive torque (ie. engine driving the prop) at reverse thrust setting (ie. negative blade angle). I was just pointing out that there can be negative torque if the negative blade angle is too small, but usually the governor will prevent any application of power while the blades are transiting through to reverse thrust blade angle.
On the Metro, negative torque is prevented by a system called NTS (Negative Torque Sensing).