Ok, I was led here from a SegaCD thread.
I was about to do this whole conversion process on "The Smurfs" because I was having trouble with SCDCONV. Right when I was opening the ISO and about to start the conversion, I noticed the .ISO attribute was set to READ-ONLY!
I removed the Read Only attribute and it converted smoothly, . . but I've yet to play it.
If anyone gets a quick black screen when trying to run SCDCONV, check if the ISO is set to READONLY.