We should never forgive. Bad actions don't suddenly become OK because the person is sorry or because time has passed. We are all accountable forever for the bad things we do.
Blackwall is held accountable though if sent to the Wardens.
He'll be poisoned by the Blight during the Joining and may not survive it. If he survives the Joining, he is basically terminally ill. Over the course of thirty years or so he will slowly die. He'll spend those years in a never ending war against the darkspawn (war is hell) and may fall in battle against them. Even if survives all those battles, he will eventually get to the point that he will begin to succumb to the Blight. His choice then will be either to go off and die a heroic death in the Deep Roads, dying in battle against the darkspawn, or slowly turning into a Ghoul.