Those are civilian model private helicopters. Rather inexpensive, and underpowered. My guess is you had some private pilots goofing off or experienced difficulty.
In the mid 1990s, my family was visiting China Camp State Park on the shore of San Pablo Bay, about fifteen miles north of San Francisco. It was late in the evening of a clear summer day, the sun just setting, but still plenty of light. I was accompanied by my wife, my two young sons, and my in-laws (my father in law was a well known and respected classical musician, and my graduate education had been centered on parapsychological research--we were pretty reliable observers of small details). I had followed UFO research closely for many years for a number of reasons, including having relatives working for the government whose work involved the subject (they were very tight-lipped, but the occasional carefully worded comment left the rest of us with little doubt they were dealing with real phenomena). My local library and the Borders store nearby kept a good supply of recent books about UFOs, and I was well aware of the appearance of black helicopters in tandem with a great many UFO sightings. As we stood there on a low cliff at the edge of the water, we became aware of the noise of helicopters coming from the northeast. Looking out over the bay, I saw what appeared to be two helicopters at least five miles away, and they seemed to be headed straight in our direction. They were coming in a direct line from the Mare Island Naval Shipyard, which we could see from our location. Mare Island had been a major nuclear submarine base prior to the opening of the base at Hood Canal off the Olympic Peninsula sometime in the late 70s or early 1980s (Mare Island closed down within a year of this incident, but was still very much in service at that time). We all stood there watching the helicopters approach; they were flying literally only a few feet above the waters of the Bay. They appeared to be coal black, and devoid of discernible markings of any kind, no letters, symbols, etc...As it turned out, their trajectory took them directly over our heads. They pulled up their noses slightly as they approached the cliff, and passed no more than fifteen to twenty feet over our heads, the prop wash blowing leaves and small sticks around. And it was then I could tell that the copters, which would have appeared just plain black to anyone, were actually a very deep charcoal grey. The reason that I could make this distinction? Between the landing runners, in the center of the underside of each copter, was the small logo "U.S.Navy",and beneath that, a serial number. The printing was in a black so deep that it was reminiscent of Carbon 60. Yet, even so, we had to look carefully to make out the figures, since there was so little difference between the tone of the lettering and the copters' paint job. Had we been any farther away, even thirty feet below, we could not have made out the markings. They were completely invisible to anyone who was not directly under, and very close to, the copters. A light bulb went off in my brain at that point. This was probably why people described black unmarked copters, because the markings were meant to be invisible to anyone who was not directly beneath, and very, very close. I believe that these were likely in the service of Naval Intelligence, which, on the basis of the particulars of my previously mentioned family connections (all sadly deceased now), I believe to be one of the agencies most deeply involved in the UFO phenomenon.
The bottom one is white. The picture is too blurry to see markings.
These images resemble Robinson helicopters. In my Oregon home area, robinson helicopters are used for flight training, tourist flights, and monitoring federal and state lands, so they are familiar in the skies. They do provide police heicopters, as well.https://en.wikipedia.org/wiki/Robinson_Helicopter_Company