I immediately thought "Light" when I read Karma. But perhaps: "Bad Karma" equals Darkness?
If we're talking thematically, this spell is somewhat similar to a Jade Shield, in that it's "reflecting" damage.
I like the idea...but what happens if the card is stolen/destroyed? If I nab Karma off of you two turns in, does any damage follow the card, so I can use it against you? If it's destroyed, is the damage released before it's time or does it just go pfft?
Just wondering.