The Comanche vented its turbine exhaust into the wash from the tail rotor, iirc, to make it harder to spot. It also used a 5-blade rotor instead of the traditional 2-blade rotor in order to make it quieter. There's a bunch of cool things we could do to make helicopters more stealthy if we cared enough.
My recollection is that the contemporary reporting was pretty specific about what it was: an existing chopper with stealth tech grafted on. Something about that (the weight or shape) reduced lift and led to a crash landing during the mission.
Some Google around NYTimes would probably turn up the details.
The story I heard at the time was that it was a combination of the stealth helicopters having lousy low-hover performance, and that during training they'd mocked a nearby solid wall up with a chain-link fence, which led to picking a poor landing zone.
And if you look at the US Army's Future Vertical Lift competition, [0] you can see some indications that those techs might make it into our future helicopters.
