I was on my 10.04 machine this evening when I decided to run an update. The packages were from the flexiondotorg ppa which is for backports of Shotwell. Shotwell had an update 2days ago so I was surprised to see more. They were for libglib2.0 and libc-bin. After logging out I got the blank screen. Could these have knackered the machine?
I'm assuming that you have used this AU question and answer.
The easiest way to confirm this is to either boot into recovery mode or CTRL+ALT+F1 for a TTY
Then run a ppa-purge on the type PPAs
This will downgrade gstreamer and shotwell back to the standard repository versions.
Once done, reboot.
If you still have a black-screen, then it is not the shotwell/gstreamer PPAs that have caused your black-screen issue.