Several users report a blank screen after installing Mac OS X 10.6, Snow Leopard. However, one suggested fix is to find a flashlight to read the screen.
In the Apple Discussions topic Display blank 24hrs after SL install, as with another describing a "very faint screen," MacBook Pro users report that the backlight just goes out during the installation.
Some posters suggest that the display went to sleep during the installation and then it didn't "wake up" with the necessary brightness for humans to see the screen.
One answer is to find a flashlight.
I had a similar issue on my MacBook Pro. I was able to complete the installation by using a flashlight on the screen. My problem was that the screen was very very dim that it looked blank. I also attempted to increase the brightness of the display, but that didn't work. Once I used the flashlight I was able to select my language and locate the mouse so I could select accept. The installation completed once that was done.
In addition, you can try increasing the brightness. F2 is the usual keyboard command for increasing brightness. Depending on your config, you may have to hold down the Fn key to get this Function key to work.
Another post points to the Energy Saver settings as the culprit:
My installation was on a MacBook Pro Core 2 Duo. After I finally completed the installation, I had to change the screen brightness setting and manually change the gamma in the calibration of the display to 2.2. I now picture the Snow Leopard holding a high-end flashlight in its mouth.
It is possible that this problem could have been avoided if the display preferences in Energy Saver had been set, before the installation began, to never have the display sleep. My display went dark and did not recover after about the same amount of time for which sleep was set.
So, if you're heading into an install of Snow Leopard, it might be best to manually turn all of the display sleep settings to Never (and maybe do the same for system sleep as well as unchecking the hard disk sleep box). Of course, it's never wise to install systemware when running on battery power. But anything can happen, or the installer may decide you're running on the Power Adapter settings.