Unity blit() not writing to RenderTexture

I found a nice Gaussian Blur shader online and am attempting to use it to blur a render texture using blit(). However, the render texture remains unblurred. I know for a fact the blur shader works as I've used it to blur the main camera, but it just doesn't seem to want to blit() my render texture.

C# Blur Script:

 void Update()
 {
     if (_material == null)
     {
         _material = new Material(_shader);
         _material.hideFlags = HideFlags.HideAndDontSave;
     }

     RenderTexture rt1, rt2;

     if (_downSampleMode == DownSampleMode.Half)
     {
         rt1 = RenderTexture.GetTemporary(source.width / 2, source.height / 2);
         rt2 = RenderTexture.GetTemporary(source.width / 2, source.height / 2);
         Graphics.Blit(source, rt1);
     }
     else if (_downSampleMode == DownSampleMode.Quarter)
     {
         rt1 = RenderTexture.GetTemporary(source.width / 4, source.height / 4);
         rt2 = RenderTexture.GetTemporary(source.width / 4, source.height / 4);
         Graphics.Blit(source, rt1, _material, 0);
     }
     else
     {
         rt1 = RenderTexture.GetTemporary(source.width, source.height);
         rt2 = RenderTexture.GetTemporary(source.width, source.height);
         Graphics.Blit(source, rt1);
     }

     for (var i = 0; i < _iteration; i++)
     {
         Graphics.Blit(rt1, rt2, _material, 1);
         Graphics.Blit(rt2, rt1, _material, 2);
     }

     Graphics.Blit(rt1, source);

     RenderTexture.ReleaseTemporary(rt1);
     RenderTexture.ReleaseTemporary(rt2);
}