When using Graphics.Blit() in Android, getting a black screen

I'm performing some GPU computation (using surface shaders) on Android (Pro) in Unity (Pro) - a series of straightforward pixel comparisons, followed by a merging of all changes observed between the textures over the previous sixty frames. To extract the results from the GPU, I was using a secondary camera and active RenderTexture, but was advised to use Graphics.Blit() instead for performance benefits (and to deal with offset issues that arose when it came to the merging shader). I used ReadPixels in both approaches to extract the data to a Texture2D.

Employing Graphics.Blit() did provide a performance benefit in the Windows Editor, and also solved the issue I'd been having with the merging step, with the program running exactly as intended. When I deployed the project on a test platform (Samsung Galaxy S4), however, all I got was a black screen with a frames-per-second overlay.

In attempting to resolve this issue, I've searched the forum pretty thoroughly for similar issues, and found a few. The most common suggestion to folks with similar problems was to add

ZTest Always Cull Off ZWrite Off Fog { Mode Off }

to the shaders, which I've done to no avail. I've used both the default renderer.material and a separate purpose-specific material to hold the textures and shaders, with it making no difference. I've attempted to process the Blit with shader to an intermediate RenderTexture before doing a straight copy of that onto the target RenderTexture (which was a proposed solution for iOS developers with a similar problem), but it didn't help.

To give an example of the context, here's one of the comparison calls:

     Texture2D cam_feed_grayscale;
     Texture2D cam_feed;
     Texture2D last_cam_feed_grayscale;
     Texture2D difference;
     RenderTexture dataPasser;
     float threshold;
     
     void Update(){
          renderer.material.SetTexture("_grayScale",cam_feed_grayscale);
          renderer.material.SetTexture("_lastFrame",last_cam_feed_grayscale);
          renderer.material.SetFloat("_threshold", threshold);
          Graphics.Blit (cam_feed, dataPasser, renderer.material);
                 
          difference.ReadPixels(shaderRegion, 0, 0);
          difference.Apply();}

where the appropriate variables are declared and new'd elsewhere, and its associated shader:

 Shader "Custom/FrameCompare" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _grayScale ("Base (RGB)", 2D) = "white" {}
         _lastFrame ("Base (RGB)", 2D) = "white" {}
         _threshold ("_threshold", Float) = 0.0
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         ZTest Always Cull Off ZWrite Off Fog { Mode Off }
         
         CGPROGRAM
         #pragma surface surf Lambert
 
         sampler2D _MainTex;
         sampler2D _lastFrame;
         sampler2D _grayScale;
         
         float _threshold;
 
         struct Input {
             float2 uv_MainTex;
         };
 
         void surf (Input IN, inout SurfaceOutput o) {
             
             half2 temp = IN.uv_MainTex;
             
             temp.x = 1.0-temp.x;
         
             half4 c = tex2D (_grayScale, temp);
             
             half4 d = tex2D (_lastFrame, temp);
             
             if(c.r < d.r - _threshold)
             {
                 o.Albedo = half3(0,0,0);
             }
             else if(c.r > d.r + _threshold)
             {
                 o.Albedo = half3(0,0,0);
             }
             else
             {
                 o.Albedo = half3(1,1,1);
             }
             
             o.Alpha = 1;
             
                         
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

As I say, everything works fine in the Editor. It's only when I deploy the code to an Android device that things seem to get a little squiffy. I suspect I'm doing something wrong with my application of Graphics.Blit(), or perhaps within the shader itself (though not to a degree that causes complaint in the Editor), but if I am I can't see what it is - any advice would be gratefully appreciated.