Well, if I were in the military, doing training exercises in an unmarked military helicopter in a relatively remote locale, I certainly wouldn’t pass on the opportunity to mess with somebody that happened by.
I think if I didn’t want to be noticed, I’d fly a civilian helicopter with normal markings. No one would give me a second look. People tend to notice things that are unusual, and a black heli with no markings is unusual.
Oh… I do fly a civilian helicopter with normal markings. I assure all of you I’m not up to anything covert. Take no notice. Just a normal helicopter, out for a ride. Trust me.
I personally saw and walked around two Hueys at a nearby air base that were completely unmarked - just green camo. They weren’t up to anything nefarious, and were sitting out where all could see. There did not appear to be any armamments on them at all, or any other strange “canisters” or anything. And an hour later I heard both take off, and watched them go.
Now given that the topic of illegality of unmarked aircraft has been covered before, it seems that at the least they weren’t legal to fly without markings, regardless of whatever else people may have been doing with them.
Some military helicopters (usually US Army) are painted a matte dark green, easily mistaken for black. Also, the turbine exhaust commonly deposits sooty crap all over the after half of helicopters, making what little “subdued” markings there are on them indistinguishable. US Army helicopters have used subdues markings since the later 1960. Early in the Vietnam “experience”, UH-1s and others were painted a glossy olive green with large red, white and blue “star and bars” insignia along with equally prominent “U.S. ARMY” in white, wide yellow tail rotor warning bands, yellow “rescue” arrows, etc. The VC and NVA were very thankful for being provided with such distinct aiming points. After losing quite a few helicopters and crews, this scheme was finally replaced with matte green paint and small black letters saying either “U.S. Army” or “United States Army.” This was about the same time white name tapes over right breast pockets and black-edged yellow enlisted stripes disappeared from field use.
US Navy and Marine helicopters currently use (as a standard scheme)a matte grey scheme with markings in a slightly diffeent shade of grey. This includes the “star and bars” national insignia that the Army gave up in Vietnam and still hasn’t revived. Between Vietnam and Desert Storm, the Marines used a very dark green and grey paint scheme which looked like hell. Some Marine UH-1s were painted in various tan and brown schemes, but it turned out that haze grey works at least as well in the desert as it does at sea.
The FBI and other assorted agencies use helicopters which are often gloss dark blue, sometimes with white roofs (called “solar caps”). These dark blue helicopters could be mistaken for black.
I believe US Customs is the only government operator of genuine flat black helicopters.
There have at times been experiments with reduced-noise helicopters. Some have been used on operational trials. I am not a helicopter engineer, but my understanding of it is that very roughly, the noise is a function of the work (lift, thrust, whatever) of the machine. You can soften the sound some, but doing so reduces the useful load and speed and so on of the machine.