Brian, the Blackhawk doesn't have a sprag clutch to allow the transmission to be disconnected. It has an APU and so doesn't need one.
It can be fitted with a transmission lock or a rotor brake (the ones I flew had no rotor brake; I found this a disadvantage on such a big aircraft). The transmission lock was only ever engaged to prevent the rotors turning during non-fuelled compressor washes. It required the ground crew to turn the rotors until a dog engaged. The engines were then spun up for washing using the APU.
The Puma no longer has an engine de-clutch facility either, as I explained earlier; only a rotor brake.
BTW, on the S-76 the hydraulics don't work with the rotor brake holding the transmission.