Material is blending with background problem

I'm making a healthbar by using a material and Graphics.Drawtexture. My material has the 'Transparent/Cutout/Diffuse' shader to make the healthbar cutoff whenever the player looses health. Now I'm having this issue where the healthbar blends with the background instead of displaying it as a solid bar. I must have something to do with the blendmode I guess, but I have no idea how to change that. So how can I fix this? Here's a screenshot: http://i62.tinypic.com/2ptbig2.jpg

For the texture I use a red texture with a black and white gradient as an alpha channel.

I tried to use a plain red texture without alpha channel and a material with the 'Diffuse' shader to test it, but I get the same result.

Here's the code

 public class Interface : MonoBehaviour 
 {
     public Texture2D healthBarTex;
     public Material healthBarMat;
     public float hitPoints;
 
     private GameObject player;
 
     void Awake()
     {
         player = GameObject.Find("Player");
     }
 
     void OnGUI()
     {
         hitPoints = player.GetComponent<Health>().hitPoints;
         if (Event.current.type.Equals(EventType.Repaint))
         {
             Graphics.DrawTexture(new Rect(0, 25, 500, 100), healthBarTex, healthBarMat);
             healthBarMat.SetFloat("_Cutoff", 1 - (hitPoints / 100));
         }
     }
 }