The important difference I see between an aspect and the suggestion of "magical" aspects is in fact the magic.
When Molly comes home and is about to be busted by Charity, there is mechanically no difference between invoking rebellious teenager or inherent magical talent for +2, I agree on that.
But there is a difference. While rebellious teenager would imply, that Molly knows how to move silently in her own home, knows how to stay hidden, which floorboards to avoid, and so on. There are no repercussions from this whatsoever.
Invoking inherent magical talent would allow you to hide in plain sight, veiling yourself with magic. And yes, like someone else said, I would allow this once. After that you either take a magical power (for example the [-0] one Death is proposing), or you drop your magic entirely. Since you would probably not give a character the aspect "inherent magical talent", without intending to use it one day, that is probably not an option that is going to get used very often. But intent to use magic should exclude you from the pure mortal benefit.
Most importantly, I see the [-0] power Death describes as far more than simply one free tag per scene. It is going to let you do impossible (or at least improbable) things as well. You could tag it on a survival roll to make fire without a lighter. Or like above, hiding without anything to hide behind. It is a rudimentary use of magic, not just the invoke of an aspect. Thinking about it, making it a [-1] power like Richard is proposing would make sense. In that case, I would probably do it like I described earlier, as a single spell power for 1 refresh.
It's like an artefact collector who is technically a pure mortal, but he uses magical items, and if the character concept is based on the use of magic in one way or another, he should no longer be considered a pure mortal. Even if the magic is just based on tagging an aspect.
You could mimic the cloak of shadows power with a stunt or two, and you would not lose your pure mortal status. Take cloak of shadows for the exact same mechanical benefit, and you are a supernatural character. I see the same reasoning applying here.