For whatever it is worth, I have the same problem. I have switched the compatability mode for both the 32bit and 64bit OBS shortcuts. I have a Dell with dual nVidia and Intel integrated graphics. I have edited the settings in the nVidia control panel, remains black. I use Camtasia and Debut and they both capture my screen just fine, but I would rather use the far more flexible OBS. But it seems a bit of loss.
Regards
Mike H