Hello!
If you mounted the ROM correctly, the black screen is just related to the IDE timeout, which is faaar longer than in KS 2.05 or 3.0.
It happened the same to me: I assume you have no hard disk connected to the A600.
In this case scsi.device (driving IDE controller) waits for hard disk responding for 15 secs or so.
Just wait for 30 seconds: if the screen stays black, you have a different problem; otherwise you'll see the common violet screen with disk insertion animation.
BY!