Well, that’s certainly interesting. The main rotor of most full-size helicopters rotates maybe 8 or 9 times a second. although one very large Russian prototype (never in production) was less than two times per second. The modified Black Hawk used in the Bin Laden operation may have had more and quieter blades than a BH normally would and was much more radar stealthy, but the basic blade mechanics were not thought to be anything special. I have no idea what you saw.
This one was rotating so slowly, it looked like a toy. Maybe half a rotation per second.
Growing up on Army bases, I saw helicopters all the time, and as an adult in SoCal, they were a constant presence. I know what they sound like, and I know what the blade rotation speed should look like. This thing broke all the rules.
I have no idea what you saw.
Well, I know what I saw, but I have no explanation for it. It just was. How? I have no clue.