I dont have a blackberry. But I was able to tether my Treo/palm phone sometime back, and here is a step by step.

1) Can your blackberry access the web? If so, you should know
phone number for the dial-up, your user name, and the account password.
If not, call up your subscription, they will tell you, if you can prove who you are.

2) Pair your N810 with the blackberry by the BT.

3) setup dial-up from your N810

4) if successful, when click on your N810, will wake the blackberry and get connect.

I do not have a blackberry, in a 'nut'-shell, that is what one do in tethering.

good luck

bun