In Divinity, some Radeon systems would give a black screen during the Direct3D test, but the game itself worked fine. As far as I know, that should not be a problem with the Beyond Divinity demo (I have a Radeon 9600 XP now and didn't have any problems). Did you try skipping the test? Did you shut down all non-essential programs, especially anti-virus, before installing the demo? If there was a problem during the install an un-install / re-install should help.
