How to model the sharp edge of a blade I want to make the edge of this blade thin so that it looks sharp, but how can I do this?
Should I use a cube instead of a plane or is there another way?
I would use the Mirror Modifier instead.

You can select the faces of your blade and then press I to Inset them. You can then move them some distance away from the Object Origin.



You can see the effect of the Mirror Modifier: