From memory it matters how a DVD is burned and also they always recommend to burn at slowest speed possible. Back in the olden times when I burned DVD’s I always used k3b for this task.
Are you using EFI boot? OM Lx .isos using EFI boot will boot to an interactive shell where you need to enter some bits of simple code to boot. That bug and it’s workaround is described here. Or just deselect EFI boot and use Legacy (MBR) boot.
For the black screen problem solving. Look at Display settings, especially under “Graphic Controller”. “VBoxSVGA” works here. Something else may work for you.
Also consider possibility of corrupted download or DVD burn. I always, as a habit, check checksums but I do see occasionally a corrupted download even though checksums are OK.
I’ll burn a DVD to check and see if that works. Hope something here helps.
Screen-shot of settings for working OM Lx Test VBox VM: