I just received my BC Super Black(Flat) instant touch up pen to cover a white edge of the back bottom of the iPhone and the chamfer. Some here say it etches into the aluminum, but all it did was sit on it and then easily wiped off. The white edge still showing.
IMO it didn't work. Maybe it will on a nick in the chamfer, but on an edge or flat surface, not so sure. Also, it is black. The sides of the iPhone are slate, and so it does show easily. Since the chamfered edge is black and glossy, people should purchase that pen. just for the chamfer.
The warnings on the package also state "Warning: This product contains a chemical known to the state of California to cause cancer".