Transparent Shader with Masking?

I'm trying to figure out how to do this. Basically what I want is a transparent object that will mask / block from view specific objects on a separate layer? Would this be done with a custom shader or is it possible to do this out of the box with unity?

For Example, in my scene, I put down a cube, set it to be full transparent, and I put a small sphere behind that cube. That sphere would be masked/blocked by the transparent cube, but everything else behind the cube would remain visible. If I moved the camera, or the sphere to the edge of the cube, the sphere would be partially visible but otherwise masked/blocked by the transparent cube's edges.