You know... I wonder if the black heart is not driven by your own perceptions. As noted, Cora was a psychopath and her heart was squeaky clean. Yet Snow did one bad thing and she has a spot? Maybe it's her guilt that's making her feel like she's corrupted. If Snow keeps overreacting and thinking that anything not pure Lawful Good will corrupt her, maybe it will. Likewise, maybe since Cora thought the things she did were all justified/right, her heart remained 'pure.' (Or it was because it was outside her body, but I'm not sure if I buy that.)

Reminds me of a webcomic I read, which featured a shadow-creature (later shown to be a young demon) that ate the shadows on people's hearts, killing them. The shadows were at first implied to be strictly moral corruption-- criminals had lots of shadows, for example-- but later on explained to be as subtle as guilt or regrets. Towards the end of the story, the demon comments that even the hero has a small shadow on her heart after a friend dies and the hero feels responsible, even though there was nothing she could have done.

Granted, that's probably a little more subtlety than we usually see in this show, but I like that as a concept much more than 'doing one bad thing dooms you forever.'