Blending of transparent textures

I write game for ipad. When i apply default diffuse shader to material of background and SpotLight for imitation of dark room, i get slow fps on ipad.
Then i set non-transparent background, create Projector object and set custom shader on projector material

Code (csharp):

1.  
2. Shader "Projector/New Light"
3. {  
4.     Properties
5.     {  
6.         _Color ("Main Color", Color) = (0,0,0,1)    
7.         _ShadowTex ("Cookie", 2D) = "" { TexGen ObjectLinear }
8.         //_FalloffTex ("FallOff", 2D) = "" { TexGen ObjectLinear }
9.     }
10.  
11.     Subshader
12.     {
13.         Pass
14.         {    
15.             ZWrite off
16.             Fog { Color (0, 0, 0) }
17.             Color [_Color]
18.             ColorMask RGB
19.             Alphatest Greater 0
20.             Blend DstColor Zero
21.             Offset -1, -1
22.             SetTexture [_ShadowTex]
23.             {
24.                 constantColor [_Color]
25.                 combine texture * primary, texture * primary
26.                 Matrix [_Projector]
27.             }
28.      
29.         }
30.     }
31. }
32.  

when in scene only non transparent object - all work perfectly: visible only objects, which blending with white color of projector texture, but when in scene transparent objects - no effect of projector to them. How modify this projector shader for support hiding in black color transparent sprites??