Using stencil buffer with OnRenderImage()/Blit

I am working with a modified version of a shader from this tutorial: https://youtu.be/LnAoD7hgDxw

I've added an option to write to the stencil buffer in the shader

 Properties
     {
         (...)
         _SRef("Stencil Ref", Float) = 1
         [Enum(UnityEngine.Rendering.CompareFunction)]    _SComp("Stencil Comp", Float) = 8
         [Enum(UnityEngine.Rendering.StencilOp)]    _SOp("Stencil Op", Float) = 2
     }


 SubShader
          {
             Tags{ "Queue" = "Geometry-1" }
 
             // No culling or depth
             Cull Off ZWrite Off ZTest Always
 
 
             Stencil
             {
                 Ref[_SRef]
                 Comp[_SComp]
                 Pass[_SOp]
             }

 (...)

 }

What I'm trying to achieve is for the shader to hide and reveal certain objects that write to the stencil buffer. This works well when the shader is applied to a material and put on a mesh:

But it doesn't work when I use the material in OnRenderImage() with Graphics.Blit(). I simply get the white color but the spheres are not revealed by the effect (the sphere that is supposed to be hidden gets obstructed by the effect but I assume it's because of depth).

How would I go about making the stencil effect work in OnRenderImage()?