Transparency and blending issues with custom-made shader

Hi Unity Community!

I am facing an issue involving transparency and blending. I know the theory on transparency and the rendering pipeline, but I am not an expert. The problem:

I am drawing some geometry shader custom made cubes over a textured quad in Unity:

When I change the color alpha value in the fragment shader, it looks like this: It behaves like blending color is the camera's background color (white)

So I've tried playing with the z buffer and disabling simply occlude my cubes.

Here is my [simplified] shader code:

Properties { _SpriteTex ("RGBA Texture Image", 2D) = "white" {} _Size ("Size", Range(0, 30)) = 0.5 _IndexPos ("IndexPos", Vector) = (15.0,15.0,15.0,15.0) _X("X",Float) = 15.0 _Y("Y",Float) = 15.0 _Z("Z",Float) = 15.0 _BrushSize("BrushSize",Float) = 0.05 _MinD0("MinD0",Float) = 0 _MaxD0("MaxD0",Float) = 0 _MinD1("MinD1",Float) = 0 _MaxD1("MaxD1",Float) = 0 _Alpha("Alpha", Float) = 0 }

 SubShader 
 {
     Pass
     {
         
         LOD 400

         Tags
         {
         "Queue" = "Transparent"
         "IgnoreProjector" = "True"
         "RenderType" = "Transparent"
         }
                     
         Blend SrcAlpha OneMinusSrcAlpha 
         Lighting Off
         //ZWrite Off
         Cull Off
         AlphaTest Greater 0
         
     
         CGPROGRAM
             #pragma target 5.0
             #pragma vertex VS_Main
             #pragma fragment FS_Main
             #pragma geometry GS_Main
             #include "UnityCG.cginc" 

             // **************************************************************
             // Data structures                                                *
             // **************************************************************
             
             struct VS_INPUT {
                   float4 position : POSITION;
                 float4 color: COLOR;
                 float4 normal:    NORMAL;
             };
             
             struct GS_INPUT
             {
                 float4    pos        : POSITION;
                 float3    normal    : NORMAL;
                 float2  tex0    : TEXCOORD0;
                 float4  col        : COLOR;
                 float    isBrushed : FLOAT;
             };

             struct FS_INPUT
             {
                 float4    pos        : POSITION;
                 float2  tex0    : TEXCOORD0;
                 float4  col        : COLOR;
                 float    isBrushed : FLOAT;
             };


             // **************************************************************
             // Vars                                                            *
             // **************************************************************

             float _Size;
             float4 _IndexPos;
             float _X;
             float _Y;
             float _Z;
             float _BrushSize;
             
             //*******************
             // DIMENSIONS
             //*******************

             float _MinD0;
             float _MaxD0;
             float _MinD1;
             float _MaxD1;
             float _Alpha;

             float4x4 _VP;
             Texture2D _SpriteTex;
             SamplerState sampler_SpriteTex;

             //*********************************
             // helper functions
             //*********************************

             float normaliseValue(float value, float i0, float i1, float j0, float j1)
             {
             float L = (j0 - j1) / (i0 - i1);
             return (j0 - (L * i0) + (L * value));
             }


             // Vertex Shader ------------------------------------------------
             GS_INPUT VS_Main(VS_INPUT v)
             {
                 [my geometry shader code...]
             }

             // Fragment Shader -----------------------------------------------
             float4 FS_Main(FS_INPUT input) : COLOR
             {
             
             if (input.isBrushed == 1.0)
             _Alpha=1.0;

             float4 colorReturn = float4(input.col.x*100.0, 0.0, 0.0, _Alpha);                
             return colorReturn; //ambient+diffuse*saturate(dot(Light,Norm));
         
             }
         

         ENDCG
     }

}

I read somewhere to change the rendering queue should be set to a number higher than 3000 (e.g, 3001). I did that and did not see a difference.

Any help is more than welcome!

Thanks for your help Unity Community :-)