I think in Ray Prouty's book he makes the points that, a helicopter blade stall is different from an airplane wing stall.
His point is that while an airfoil will invariably stall around 15 to 17 deg AOA, In a helicopter, however, the blade will reach maximum pitch for only a few millisecond.
that instant of time is not long enough for the stall to develop. As the result rotor wind tunnel have shown a rotor blade can reach 30 deg AOA and still not be stalled.