Try Medieval Kings Chess II. I wouldn't say that the engine is particularly strong (there are 6 levels), but the game has nice graphics and it works on Blackberries. You can also play people online, but little glitches here and there make it more annoying than enjoying. One other gripe I have is that there is no "flip board" feature, so if you play black, you'll be looking at your pieces from white's perspective.
This is the only one I've played and the engine is dissapointingly weak, given the limited processing power available I doubt you'll get too much strength out of a Blackberry but this one seemed particularily docile.
How strong is chess for Blackberry by Odesys? The free download only has one level of 8 levels available in the pay version and level 1 is weak!