Low Radar Cross Section/Stealth helicopters have been in development, in some form or another, since the early 1970's.
It looks like the Air America Hughes 500P might have had composite rotor blades which help it attain a lower RCS. Hughes did some work on a composite tail rotor blade in the mid-70's and, apparently, also prepared a Low RCS-shaped OH-6A fuselage for static testing (I am trying to get more info on this one at the moment).
In the mid-70's Sikorsky did a design study based on the UH-60 that featured various Low-RCS fuselage designs.
The ACAP helicopters flown by Bell and Sikorsky in the early 80's were early attempts at flying helicopters with certain stealthy aspects to their designs. Drawings released into the public domain show further ACAP designs with some fairly radical Low-RCS fuselage shapes.
The aforementioned EH-60 with Low-RCS shaping flew in the late 80's.
Bell also had a Low-RCS kit for their OH-58D Kiowa and a few photos are on the web showing it's modifications.
Aviation Week and Space Technology magazine ran an article on Low-RCS helicopters in February 1995. This hinted at two different stealthy rotorcraft and one of these might have been a program using the project name "TEK". Both were possibly McDonnell Douglas projects. MDHC have run at least one unacknowledged special access program in collaboration with the USAF but it is believed that this was a technology demonstrator only. What exactly the technology being demonstrated is unknown but I'd guess that it was low-observable technology.
The RAH-66 Comanche apparently had a ridiculously small RCS.
A Low-RCS Apache kit was also possibly test-flown in the early 1990's.
The photos from the aftermath of the Bin-Laden Raid appear to indicate the existence of a stealthy troop-transport helicopter.
I've sometimes wondered if that tail section was some kind of counter-intelligence plant, using an elaborate mock-up left behind at the scene to fool China and Russia and others into believing that the U.S. possess a secret helicopter. I think the Special Forces guys had enough to do to get in and out of Abbatobad unscathed, having dealt with Bin-Laden, without having to worry about bring in a fake helicopter or part of one with them.
The big aviation publications and noted aviation journalists seem to have spent all of their time chasing stealthy fixed-wing aircraft but have paid little attention to researching Low-RCS helicopters. There appear to be a few classified stealthy helicopters, past and present, that remain secret at this moment in time. Hopefully some of the older Low-RCS helicopters that employed earlier technology might be declassified at some point in the near future.