Blur the alpha part of a shader

First I explain what I want to achieve. My goal is to get a soft shadow alternative running on iOS (because unity soft shadows does not work on this plateform). I made a "transparentShadow" shader that is fully transparent but receive the shadow. Here is what I get with my shader and hard shadow :

Here is my code :

 Shader "Custom/Transparent-Shadows-blur"
 {
     Properties
     { 
         // Shadow Stuff
         _ShadowIntensity ("Shadow Intensity", Range (0, 1)) = 0.6
         _ShadowBlur ("Shadow Blur",  Range (0, 0.01)) = 0.006  
     }
  
     SubShader
     {
         Tags {
             "Queue"="AlphaTest" 
             "IgnoreProjector"="True" 
             "RenderType"="Transparent"
         }
 
         LOD 300
     
         // Shadow Pass : Adding the shadows (from Directional Light)
         // by blending the light attenuation
         Pass {
             Blend SrcAlpha OneMinusSrcAlpha 
             Name "ShadowPass"
             Tags {"LightMode" = "ForwardBase"}
               
 CGPROGRAM 
         // Upgrade NOTE: excluded shader from DX11 and Xbox360; has structs without semantics (struct v2f members lightDir)
         #pragma exclude_renderers d3d11 xbox360
         #pragma vertex vert
         #pragma fragment frag
         #pragma multi_compile_fwdbase
         #include "UnityCG.cginc"
         #include "AutoLight.cginc"
      
         struct v2f { 
             float2 uv_MainTex : TEXCOORD0;
             float4 pos : SV_POSITION;
             LIGHTING_COORDS(3,4)
             float3    lightDir;
         };
      
         float4 _MainTex_ST;
         sampler2D _MainTex;
         float _ShadowIntensity;
         float _ShadowBlur;        
      
          v2f vert (appdata_full v){
              v2f o;
             o.uv_MainTex = TRANSFORM_TEX(v.texcoord, _MainTex);
             o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
             o.lightDir = ObjSpaceLightDir( v.vertex );
             TRANSFER_VERTEX_TO_FRAGMENT(o);        
             return o;
         }
     
         float4 frag (v2f i) : COLOR
         {    
             float atten = LIGHT_ATTENUATION(i);
             half4 c;
             c.rgb =  0;                
             c.a = (1-atten) * _ShadowIntensity;
             
             return c;
         }
 ENDCG
       }
   }
   Fallback "VertexLit"
 }

I would like to blur the result of this shader to simulate soft shadows, in order to have this kind of result :

I have read a lot about blur techniques shader. But it only blur the texture (like this link text).

But I don't achieve to blur the alpha part.

Any idea please?