The Black screen can be caused by all sorts of things. A bad update that adversly affects the system. could be the dreaded KSoD or Black Screen of Death. Could jsut be that explorer.exe did not execute and needed to be manually started.
I cant really answer why it happened. As there are so many factors that go into it. You best option would be to have your Vista disk handy so you can use a restore point. Create a Restore point right now, before you install SP2. If you get the Black Screen again you can use a Restore point that you created before the install of SP2 and get your system back.
How to create a restore point in Windows Vista:
How to create a restore point
Recovering the Vista Bootloader from the DVD - NeoSmart Technologies Wiki
At step 2 choose the option "System Restore"
That way you have something to fall back on without having to reinstall the OS again.