Typically a phone is only blacklisted if it was reported stolen. If you bought it from them, talk to them to resolve it, otherwise I assume you have a stolen phone and it doesn't belong to you.
It also might be blacklisted because the original owner didn't finish payments on the phone and didn't return it to the carrier. (If they then tried to sell it, that's about the same as stealing it from the carrier anyway.) Follow L0n3N1nja's device. Thread closed.