Good idea to shut down backlight, it sucks battery quite a bit more than just playing music. How I do it on my device is by using settings in my player, PocketMusic, which allows mapping a button to toggle screen backlight and/or a timer. Since Touch has so few buttons, I've used the automatic feature...
The full featured bundle version of PocketMusic is $19.95, but there's a fair lot of features in the free version. Both are available here...
http://www.pocketmind.com/pocketmusic.htm
If you're bent on using some feature-lacking player like WMP, Sprint Music, or HTC Audio Manager, then you can install a separate app to kill the screen light. Use a search site, like
Google, to find ScreenToggle. It's
freeware, and the version for PPC2003 works nicely on Touch.