I've replicated the issue with the black screen using 3.9 GTK and I suspect it may be the same with the SDL version.
When I checked the settings, I found that (I have this on by default) autostart mode is set to Inject into RAM. when I have it as a virtual filesystem, I also get the black screen
I suspect this is also going to be the same for C64Forever also.
I will see if I can figure out what is going on with this though. Thanks for letting me know.
It may take a few days as I'm working on the next Game Jam, but I will give this some time also