Do you mean black like nothing on the screen or nothing but an x shaped mouse cursor? If you get to the x mouse cursor thats good, just give it lots of time.
Which Ubuntu distro, the one I got I got from the above site 4 days ago (6.10ppc). I ran the live install disk and installed from there. Allowed it to completely repartition its own hard drive (I disconnected the PCI card w. main system drives) so there was not a mac partition on the drive. After the install it booted fine, and surfed the web right away like a champ. The only problem I am having is i cant get the display above 800*600 yet. I think the support for the earlier mac cards is patchy, but as long as you have an ATI rage chip of any type it should work.
Go here:
http://www.lowendmac.com/imacs/index.shtml
Identify exactly what mac and video chip you have and try to specify that while installing. If you are going to spend any time spend it on Ubuntu. I found the whole SUSE bootloader whoohaa to much trouble, and fedora would rarely get into the GUI after the install.
Keep in mind even after installing it is really slow the first couple of times it boots into the gui (really,really, leave the room and start a new hobby kind of slow)if you get the X mouse keep waiting.
During parts of the Ubuntu install the screen went black a couple of times for several minutes each time. The only time I had to do a forced restart was after an install of some suplimentary hardware after booting up a couple of times. Try it again and give it WAY longer than you think you should when you think it stalls. Think of it as a meditative statement against the urgency of our modern times.
"African open source code on a hip west coast box? Sacralige."