I use Geocache Navigator with my Verizon Tour 9630. It works very well. Of course it requires your BlackBerry have a functioning GPSr. I don't think AT&T has ever blocked the GPS, but before you buy the app, be sure the GPSr works. If you don't have any other apps that use the GPSr, go to Options / Status and type the letters test. That will bring up the self-test menu. Go outdoors with a clear view of the sky and run the GPSr self-test. If if passes, you've got a functioning GPSr.
You will have to have a data plan to use Geocache Navigator. Neither it nor your BlackBerry have any preloaded maps, so all the maps have to come through your data connection in real time. That also means you won't have maps if you are geocaching in an area with no cell phone coverage. That may not matter, but you need to be aware of it.
AT&T has a history of blocking users from running apps that compete with apps they license. They block BlackBerry Maps, for example, because it competes with their Nav app. So it is entirely possible that you won't be able to run Geocache Navigator after installing it. It should have a free trial, so you can find out risk-free. If it won't run, there are work-arounds that may enable it. A discussion of those is outside the scope of this forum, but if you post your question on crackberry.com, you can get an answer there.