Goldmines are accounts that have been left unactivated by their owner for at least 7 straight days, but they can come back anytime they want if they wish so.
Blocked accounts are usually multi-accounts or, like Radooo said, very, very bad players
... and those accounts are not meant to be reactivated (yet cannot be deleted) ... that's why you can see them, but not attack htem (or they would be goldmines, therefore potentially playable accounts
)