Black screen after failed restore - G5
The resolution to my problem was somewhat messy. 

The black screen I mentioned was a result of a failed osx restore. It failed because of a bad dmg. The machine then suffered a kernel panic and I guess it was unable to roll back te changes it had made. 

I removed some ugly looking ram and cleaned te others, reinstalled osx from disk using a new drive and have so far not experienced any problems. 

Problem caused by dodgy dmg installation and dodgier ram