I like the idea, the execution is what made this not a top pick for me. If it were cumulative +1|+0 and balanced around that with a lower cost, I'd be more inclined to like it, since I like the idea of splashing an element (mono-Light with Shadows splashed in on Darkness mark). It would also be more interesting if it triggered when your opponent played a spell (possible if the buff is cumulative / until the end of your turn). It's just a bit too limited right now, and needs to have more than one use in my opinion. As it is now, it seems only effective with Angels on account of its 0 base ATK.
I also agree with OT's logic, which is the same reason I don't like the idea of 'critical strike' on cards most of the time.