Does alpha blending work on image effects? (is _MainTex alpha presevered?)

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).

Code (CSharp):

1.  
2. static const fixed4 _yellow = fixed4(1, 1, 0, 1);
3.  
4. half4 frag_composite(v2f_img i) : SV_Target {
5.      float4 s;
6.      s = tex2D(_MainTex, i.uv);
7.      s = lerp(s, _yellow, s.a);
8.      return s;
9. }
10. ENDCG
11. Subshader {
12.      Pass {    // 0
13.          ZTest Always Cull Off ZWrite Off
14.          Blend SrcAlpha OneMinusSrcAlpha
15.          CGPROGRAM
16.          #pragma vertex vert_img
17.          #pragma fragment frag_composite
18.          ENDCG
19.      }
20. }
21.  
22.  
23. private void OnRenderImage(RenderTexture src, RenderTexture dest){
24.      Graphics.Blit(src, dest,shader_compositor);
25. }

This just draws _MainTex without drawing yellow in the transparent background. The contents of _MainTex are my spaceship and star field and empty space behind.

 

Turns out alpha works fine. I was using someone else's code as a starting point and it's outdated. Still compiles and runs, annoyingly!