[haus]

One idea that comes to mind would be to carefully tape over the silver part of the back cover, then sand / rough up the shiny part, spray it black with plastic paint, then cover it a couple of times with a glossy clear coat, and see how that looks. It'll be a wear item so if it were me I'd use clear automotive wheel paint for the last steps. You could try it on a piece of plexiglass from the hardware store first to get a sense of how it might turn out.

But the other question is...does simply "covering" the camera really satisfy the security requirements? Technically you could take the back cover off and snap photos. So it may not be worth it, esp. if you get caught with a phone model that is known / presumed to have a camera.