Where to Graphics.Blit()?

I have 2 cameras, one for a video background, one rendering objects. Both have a target render texture. I want to use a material as a post-processing effect to add a global transparency to the objects (each may already have an individual transparency), so I want to blit the result of a material to screen. However when I call graphics.blit in the built-in render pipeline (in OnRenderImage) I don't get the same result as in the material preview (see pictures).

Expected result (material preview) by setting the material's main texture:

Graphics.Blit in OnRenderImage by blitting in OnImageRender:

 private void OnPreRender()
         {
             material.SetFloat(variableName, Multiplier);
             material.SetTexture(backgroundTextName, backgroundRt);
         }
 
         private void OnRenderImage(RenderTexture source, RenderTexture destination)
         {
             Graphics.Blit(source, destination, material);
         }

The shader:

 Shader "PostProcessing/AlphaBlend" 
 {
     Properties
     {
         _MainTex("Main Texture", 2D) = "white" {}
         _BackgroundTex("Background Texture", 2D) = "white" {}
         _Alpha("Alpha Multiplier", Range(0,1)) = 0.5
     }
 
 
     SubShader{
 
         Tags{ "PreviewType" = "Plane" }
 
         Pass {
                     ZWrite Off
                     Lighting Off
                     ColorMask RGBA
 
                     CGPROGRAM
                     #pragma vertex vert
                     #pragma fragment frag
                     #include "UnityCG.cginc"
 
                 sampler2D _MainTex;
                 sampler2D _BackgroundTex;
                 float _Alpha;
 
                 struct v2f {
                     float4 pos : SV_POSITION;
                     float2 uv : TEXCOORD0;
                 };
 
                 v2f vert(appdata_base v)
                 {
                     v2f o;
                     o.pos = UnityObjectToClipPos(v.vertex);
                     o.uv = v.texcoord;
                     return o;
                 }
 
                 float4 frag(v2f_img i) : COLOR
                 {
                     float4 color = tex2D(_MainTex, i.uv);
                     color.a = color.a * _Alpha;
                     float4 backgroundColor = tex2D(_BackgroundTex, i.uv);
                     float4 output = (color.a * color + (1-color.a) * backgroundColor);
                     output.a = 1;
                     return output;
                 }
                 ENDCG
             }
         }
 }

As you can see the pipe colours are very dull in the second case. I have tried to blit to a different render texture, then blit that render texture in Update, LateUpdate, but in those cases I get a black screen.

Does someone understand why blitting in OnImageRender results in dull colors (more transparency than I'd expect)? Where/how can I use my material since it does what I want given the preview?