Garmin for Blackberry might be different. I believe all the maps are downloaded to the media card on the Blackberry. So, it only relies on the GPS, not cell towers and data availability. Don't quote me on all this, it's just a guess.
If it does rely on cell towers, then I agree with Dubdub. Get a standalone GPS device.
Here is the link: https://my.garmin.com/mygarmin/marketing/mobile.htm