The phone only draws the amount of current it needs. The CURRENT RATING of the charger doesn't matter. If it's too LOW it will charge slowly, if it's too high it will only take what it needs. Most cell phone chargers max out at 1AMP, usually less. The Blackberry (or any other cell charger) will be fine. I've used "universal USB chargers" and AC adapters rated as high as 2 amps on a phone that came with a 500ma charger for years.
not exactly; the phone doesn't just take what it needs. Electricity flows as it will, going to the least resistant path to ground. If the water dam can't stop the overwhelming force of a flood, it destroys the community.
If I plugged the USB cable directly to an outlet (let's just say it's also DC for the sake of the point), the little resistors on the phone aren't going to stop the potential force of 120 volts and or the current flow of 15-20 amps.
Now granted, you're not going to experience that potential force or amp draw... you shouldn't get anything higher than the 4.5 or whatever volts the USB standard provides... but if you charged the battery too fast (higher current flow), you could overheat and destroy it (potentially with an explosion too).
Regardless, any legitimate aftermarket charger (or OEM one from BB), is pretty unlikely to cause any problems, and will work exactly as it it should. HOWEVER, only the actual OEM charger is guaranteed not to fry your phone... under it's warranty period, that is. heh.