I did some tests on the HV 20 to see the differences in black levels between the Cinemode and the Regular mode gammas. I did this to see whether the blackest black in the Cinemode was truly black.

To create the tests, I first closed the iris as much as possible and then capped the lens to eliminate any light. I then captured direct into Premiere Pro. I then used the eyedropper within the color correct tools to measure R,G,B, levels.

Default video gamma in TV mode: 0,0,0, RGB

Cinemode Gamma: 3,3,3 RGB

With my camera, it seems as if the cinemode is raising the black level slightly. It will not produce a black below 3,3,3.

it seems as if the cinemode is raising the black level slightly. It will not produce a black below 3,3,3.

I noticed this on the first cinemode footage posted here, although I expected you might be able to slide it down into true black by using some of the other settings on the camera. I kind of figured you'd be able to drop the gray-blacks down to black-black in cinemode by either the exposure lock (cranking down the exposure) and/or by lowering the brightness setting under custom settings, or maybe even bumping up the contrast settings there (although I wondered if upping the contrast would defeat the extended latitude that cinemode provides).

It sounds like you tried cranking down the aperture by adjusting the exposure lock, how about lowering the brightness setting, did you try that?