Does alpha blending work on image effects?

I've two cameras rendering my background and foreground separately, and want to composite them with an image effect. This is working except that the alpha value for the foreground provided as 'src' in the OnRenderImage() is always 1, despite setting the camera's clear flags to (0,0,0,0).

 static const fixed4 _yellow = fixed4(1, 1, 0, 1);

 half4 frag_composite(v2f_img i) : SV_Target {
     float4 s;
     s = tex2D(_MainTex, i.uv);
     s = lerp(s, _yellow , s.a);
     return s;
 }

 ENDCG

 Subshader {
     Pass {    // 0, quarter size
         ZTest Always Cull Off ZWrite Off 
         Blend SrcAlpha OneMinusSrcAlpha
         CGPROGRAM
         #pragma vertex vert_img
         #pragma fragment frag_composite
         ENDCG
     }
 }

 private void OnRenderImage(RenderTexture src, RenderTexture dest){
 Graphics.Blit(src, dest,shader_compositor);
 }

This just draws _MainTex without drawing yellow in the transparent background.