Inverted flight in real helicopters is a neat parlor trick, and many modern helos are capable of brief encounters with negative G, but none are capable of continuous inverted flight due to a variety of reasons. Having flown and demonstrated negative G and inverted maneuvers, here is my read on it. (Caution, in many helicopters, negative G can result in loss of control or rotor failure) :
1) Engine and fuel systems are not designed to maintain proper fuel and oil flows, so the fuel pressure and oil pressure lights come on almost immediately. If positive G not restored in a few seconds, fuel or oil starvation will result in engine shutdown or internal failure.
2) Main rotor control is the sum of the thrust and the moment the rotor produces. As thrust is reduced, control effectiveness diminishes, to nil in many rotor designs. If there is sufficient hinge offset (the distance of the flapping hinge from the mast, expressed as a percentage of the rotor radius) the rotor can still be effective at negative G. For high offset rotors (such as the BO-105), -1 G is possible. For the Black Hawk and S-76, about -.5 to -.6 G is all you can reliably get to. I used to demonstrate -.5 G with snappy roll reversals in the armed S-76 and in Black Hawks.
For teetering rotors, like the older Bell family, the Tilt Rotors and the Robinson, fill out all your personal paperwork and settle all debts before trying anything like low G, because you won't have to worry about the landing.
Comanche can get very negative, because it has a high offset main rotor, but its fuel system is not designed for it. There really is no mission need to perform extremely low G for any length of time.