There is not much you can do, Chris. Once the device is blacklisted, only the person (or company) who blacklisted it in the first place can take it off the list.
Likely, the person who sold you the phone did NOT pay his/her remaining Tab and Koodo blacklisted the phone when selling the delinquent account to collections.
You're out of luck on this one. Sorry :(
EDIT: What I mean by "out of luck" is that the blacklisted phone you have has been rendered useless. You may be able to use it as a small tablet or a music player, but all phone functions, including SMS, won't be useable again with any Canadian carrier. WiFi will still work, though.