In my experience, DGEN can work fine for one or two ROMs and, when starting a third ROM, the screen may stay black – even when using HDMI video! For me, the only way to solve this temporarily is to restart the RPi.
I added the Genesis libretro core “Picodrive” to the RetroPie Setup Script. I would suggest to try this. It does not come with the SD-card image, but using the binaries-based installation will install and configure it on your RPi.