In play mode blur material getting weird. can I fix this?

I am using a blur material in a cube and I put the cameras in so I will get the blurry effect when I want. So I have a problem using 1 blur material that getting the play screen upside down. If I am using 2 of them is fine. At scene everything is fine but in play mode something gets wrong and more of the half screen is black for some reason. Do we know why is that?.. When I export it everything will be fine or not?

Here is the shader:

 ![Shader "Custom/WaterBlur" {
     Properties {
     _blurSizeXY("BlurSizeXY", Range(0,10)) = 0
 }
     SubShader {
 
         Cull off
         // Draw ourselves after all opaque geometry
         Tags { "Queue" = "Transparent" }
 
         // Grab the screen behind the object into _GrabTexture
         GrabPass { }
 
         // Render the object with the texture generated above
         Pass {
 
 
 CGPROGRAM
 #pragma debug
 #pragma vertex vert
 #pragma fragment frag 
 #pragma target 3.0
 
             sampler2D _GrabTexture : register(s0);
             float _blurSizeXY;
 
 struct data {
 
     float4 vertex : POSITION;
 
     float3 normal : NORMAL;
 
 };
 
  
 
 struct v2f {
 
     float4 position : POSITION;
 
     float4 screenPos : TEXCOORD0;
 
 };
 
  
 
 v2f vert(data i){
 
     v2f o;
 
     o.position = mul(UNITY_MATRIX_MVP, i.vertex);
 
     o.screenPos = o.position;
 
     return o;
 
 }
 
  
 
 half4 frag( v2f i ) : COLOR
 
 {
 
     float2 screenPos = i.screenPos.xy / i.screenPos.w;
     float depth= _blurSizeXY*0.0005;
 
     screenPos.x = (screenPos.x + 1) * 0.5;
 
     screenPos.y = 1-(screenPos.y + 1) * 0.5;
 
     half4 sum = half4(0.0h,0.0h,0.0h,0.0h);   
     sum += tex2D( _GrabTexture, float2(screenPos.x-5.0 * depth, screenPos.y+5.0 * depth)) * 0.025;    
     sum += tex2D( _GrabTexture, float2(screenPos.x+5.0 * depth, screenPos.y-5.0 * depth)) * 0.025;
     
     sum += tex2D( _GrabTexture, float2(screenPos.x-4.0 * depth, screenPos.y+4.0 * depth)) * 0.05;
     sum += tex2D( _GrabTexture, float2(screenPos.x+4.0 * depth, screenPos.y-4.0 * depth)) * 0.05;
 
     
     sum += tex2D( _GrabTexture, float2(screenPos.x-3.0 * depth, screenPos.y+3.0 * depth)) * 0.09;
     sum += tex2D( _GrabTexture, float2(screenPos.x+3.0 * depth, screenPos.y-3.0 * depth)) * 0.09;
     
     sum += tex2D( _GrabTexture, float2(screenPos.x-2.0 * depth, screenPos.y+2.0 * depth)) * 0.12;
     sum += tex2D( _GrabTexture, float2(screenPos.x+2.0 * depth, screenPos.y-2.0 * depth)) * 0.12;
     
     sum += tex2D( _GrabTexture, float2(screenPos.x-1.0 * depth, screenPos.y+1.0 * depth)) *  0.15;
     sum += tex2D( _GrabTexture, float2(screenPos.x+1.0 * depth, screenPos.y-1.0 * depth)) *  0.15;
     
     
 
     sum += tex2D( _GrabTexture, screenPos-5.0 * depth) * 0.025;    
     sum += tex2D( _GrabTexture, screenPos-4.0 * depth) * 0.05;
     sum += tex2D( _GrabTexture, screenPos-3.0 * depth) * 0.09;
     sum += tex2D( _GrabTexture, screenPos-2.0 * depth) * 0.12;
     sum += tex2D( _GrabTexture, screenPos-1.0 * depth) * 0.15;    
     sum += tex2D( _GrabTexture, screenPos) * 0.16; 
     sum += tex2D( _GrabTexture, screenPos+5.0 * depth) * 0.15;
     sum += tex2D( _GrabTexture, screenPos+4.0 * depth) * 0.12;
     sum += tex2D( _GrabTexture, screenPos+3.0 * depth) * 0.09;
     sum += tex2D( _GrabTexture, screenPos+2.0 * depth) * 0.05;
     sum += tex2D( _GrabTexture, screenPos+1.0 * depth) * 0.025;
        
     return sum/2;
 
 }
 ENDCG
         }
     }
 
 Fallback Off
 }][1] 

[1]: /storage/temp/65808-screen-shot-2016-03-13-at-163020.png