Is it possible access the contents of a destination buffer from a Blit?

Unity Engine
,
1

Given

Graphics.Blit(src, dest, material, pass);

…is it possible to do something like…

sampler2D _MainTex;
sampler2D _OutputTex;

fixed4 frag (v2f i) : SV_Target{
fixed4 output = tex2D(_MainTex, i.uv);
fixed4 secondary = tex2D(_OutputTex, i.uv);
output = lerp(output, secondary, 0.5);
return output;
}```

...and composite the src over the existing dest contents?

This could be used for special effects like accumulating multiple frames for a temporal motion blur.