In my playthrough I romanced blackwall and the revelation was hard, but to me the question of rescuing him wasn't even in question. The man showed honor, bravery, and a strong sense of right and wrong throughout his service to the inquisition. He faced down demons, bandits, giant beasts, and even played a major role in saving the wardens of adamant from all being lost. He did it all, without a care for himself and always thought of how our actions would affect others, always concerned with innocent casualties and justice.
To me, blackwalls past merely gave reason for his current point of view, and that he was already reformed, already attoned through actual life threatening action. I saved him from prison and did what should have happened, gave him to the warden order that he helped save. The one he reminded what it meant to be a true warden. And I continued the romance even with thwt decision. Whatever happened in the future would be then, and for now, he and my warden would work to save the world together.