Render sprite only on top of a specific sorting layer

We're trying to implement a simple blob shadow in our 2D game, but the way the scene is laid out, we have the blob shadow going over certain objects in the "background" layer. We were wondering if having a ground layer and having the blob shadow sprite render only in objects that are in that ground layer and not rendering in any other layer would be possible. Is this something that could be done by writing a shader for the blob shadow and using sorting layers instead of render queue? Thanks!