From what I've been told, there are only three ways a phone can be blacklisted.
1) It was purchased on an installment plan that wasn't paid. (Next)
2) It is active on a postpaid plan that wasn't paid. (Contract)
3) It was requested by the account manager, because it was stolen.
In the first case, you would be in trouble. It would very likely be blacklisted.
In the second case, only devices that are active on the account in question at the time of nonpayment will be blacklisted. So, for example, if you activate the phone on your account and the original owner doesn't pay his bill, then you're fine, because it would blacklist only whatever devices were active on his account, and the phone is on your account now and not his.
In the third case, I believe you can only report a phone as stolen if it is active on your account. So, that shouldn't be an issue.
If it isn't currently blacklisted, and you are able to activate it on your account, then you won't have anything to worry about. You could call AT&T and ask for clarification, though, so you feel better about it, but you should be safe.