SDL_SetGamma not fully black at 0.0

I’m trying to use SDL_SetGamma to make my desktop flicker to complete
black at random intervals. (I find it to be relaxing on my eyes).

I can get it working if I open up a window using SDL_SetVideoMode first,
then call SDL_SetGamma(0.0, 0.0, 0.0). The screen does go completely
black just like I want. The problem with this is that when my SDL window
loses focus it stops doing the flickering (usually). I’m using X11, and
it only
stops flickering whenever I put the focus on an X11 application (focusing
on a terminal doesn’t stop it for some reason).

I can also get SDL_SetGamma to set the gamma without loading a window
first; just by calling SDL_Init to initialize the video. This has the
advantage that it doesn’t open a windown, so it can’t lose focus, so it
keeps flickering, whatever application I open. However, even though I
call SDL_SetGamma(0.0, 0.0, 0.0), the screen doesn’t go completely black,
but is actually, I think, at 0.1, 0.1, 0.1. I think this because
when using
the command-line tool “xgamma”, 0.1 is the minimum gamma value.

Is there some trick I can do, either with the window, or without, that
will get the gamma to actually go to 0.0, and keep flickering in the
background even after I select other windows?

Thank you, I’ve spent several forlorn hours trying to figure this out.