How to alter/modify a RenderTexture with a shader,How can I modify/"post process" a RenderTexture using a shader

Hi, I'm using a RenderTexture and I need it to fade it continually with a color.

I'm not sure if I'm using Blit correctly and/or if it can be used for this purpose, here's my code:

 // - BumpTexture is the RenderTexture
 // - blendColor is the targeted Color for the blending
 // - ReturnToNormalMat is a material containing the shader
 
         this.ReturnToNormalMat.SetTexture("_MainTex", this.BumpTexture);
         this.ReturnToNormalMat.SetColor("_Color", blendColor);
         this.ReturnToNormalMat.SetFloat("_LerpValue", lerpValue);
         Graphics.Blit (this.BumpTexture, this.BumpTexture, this.ReturnToNormalMat);

Shader:

 Shader "Hidden/LerpToColor"
 {
     Properties
     {
         _MainTex ("Texture", 2D) = "white" {}
         _LerpValue ("Lerp Value", Range (0, 1)) = 0.1
         _Color ("Color", Color) = (0.5,0.5,0.5,1.000000)
     }
     SubShader
     {
         // No culling or depth
         Lighting Off Cull Off ZTest Always ZWrite Off
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 vertex : SV_POSITION;
             };
 
             v2f vert (appdata v)
             {
                 v2f o;
                 o.vertex = UnityObjectToClipPos(v.vertex);
                 o.uv = v.uv;
                 return o;
             }
             
             sampler2D     _MainTex;
             float4        _Color;
             float        _LerpValue;
 
             fixed4 frag (v2f i) : SV_Target
             {
                 fixed4 col = tex2D(_MainTex, i.uv);
                 return lerp(col, _Color, _LerpValue);
             }
             ENDCG
         }
     }
 }

What I get is that the renderTexture turns black, "col" seems to always be black. If I return for instance float4(col.r, col.g, col.b, 1) I get black. Maybe it has to do with how uvs are set ?

I could convert the renderTexture to a Texture2D and use GetPixels/SetPixels but it sounds like an overkill ? Thanks !