Blend 2 Texture with alpha

I try to do a shader that blend 2 texture and have alpha channel.

 Shader "Blend 2 Textures + Alpha" { 
     
 Properties {
     _Color ("Color Tint (A = Opacity)", Color) = (1,1,1,1)
     _Blend ("Blend", Range (0,1)) = 0.0 
     _MainTexture1 ("Texture1", 2D) = "" 
     _MainTexture2 ("Texture2", 2D) = ""
     _Mask ("Culling Mask", 2D) = "white" {}
     _Cutoff ("Alpha cutoff", Range (0,1)) = 0.1
 }
 
 Category {
     Material {
         Ambient[_Color]
         Diffuse[_Color]
     }
 
     // iPhone 3GS and later
     SubShader {
     Tags {"Queue" = "Transparent"}
     ZWrite On
     Cull Off
     Blend SrcAlpha OneMinusSrcAlpha
     AlphaTest GEqual [_Cutoff]
      
     
     Pass {
         Lighting On
         
         SetTexture[_MainTexture1]
         SetTexture[_MainTexture2] { 
             ConstantColor (0,0,0, [_Blend]) 
             Combine texture Lerp(constant) previous
         }
         SetTexture [_Mask] {combine texture * previous}
         SetTexture[_Color] {Combine previous * constant ConstantColor[_Color]}
         
         
     }}
 }}

This shader is similar to this:

 Shader "MaskedTexture"
 {
    Properties
    {
       _Color ("Color Tint (A = Opacity)", Color) = (1,1,1,1)
       _MainTex ("Base (RGB)", 2D) = "white" {}
       _Mask ("Culling Mask", 2D) = "white" {}
       _Cutoff ("Alpha cutoff", Range (0,1)) = 0.1
    }
    SubShader
    {
       Tags {"Queue"="Transparent"}
       Lighting Off
       ZWrite Off
       Blend SrcAlpha OneMinusSrcAlpha
       AlphaTest GEqual [_Cutoff]
       Pass
       {
          SetTexture [_Mask] {combine texture}
          SetTexture [_MainTex] {combine texture, previous}
          SetTexture[_MainTex] {Combine previous * constant ConstantColor[_Color]}
       }
    }
 }

Second shader work perfectly. I have problem with alpha channel of the first shader. There is a black line along border. I don't understand. Somebody can help me?