Question 2D sprites with black backfaces.

I'm almost comepletely newbie to Unity. So I wanted to make a new shader that would draw pure black backface of 2D sprites. This is what I have now:

Code (CSharp):

1. Shader "Custom/NewSpriteShader"
2. {
3.     Properties
4.     {
5.         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
6.         _Color ("Tint", Color) = (1,1,1,1)
7.         [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
8.         [HideInInspector] _RendererColor ("RendererColor", Color) = (1,1,1,1)
9.         [HideInInspector] _Flip ("Flip", Vector) = (1,1,1,1)
10.         [PerRendererData] _AlphaTex ("External Alpha", 2D) = "white" {}
11.         [PerRendererData] _EnableExternalAlpha ("Enable External Alpha", Float) = 0
12.     }
13.  
14.     SubShader
15.     {
16.         Tags
17.         {
18.             "Queue"="Transparent"
19.             "IgnoreProjector"="True"
20.             "RenderType"="Opaque"
21.             "PerformanceChecks"="False"
22.             "PreviewType"="Plane"
23.             "CanUseSpriteAtlas"="True"
24.         }
25.  
26.         Lighting Off
27.         ZWrite Off
28.         Blend SrcAlpha OneMinusSrcAlpha
29.         Pass
30.         {
31.             Cull Front
32.             Color (0,0,0,1)
33.         }
34.         Pass
35.         {
36.         CGPROGRAM
37.             #pragma vertex SpriteVert
38.             #pragma fragment SpriteFrag
39.             #pragma target 2.0
40.             #pragma multi_compile_instancing
41.             #pragma multi_compile_local _ PIXELSNAP_ON
42.             #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
43.             #include "UnitySprites.cginc"
44.         ENDCG
45.         }
46.     }
47. }

Basically, it's a default sprite shader with

Cull Front

feature. However, it doesn't work brilliant and it draws backfaces very roughly, like a chunk of polygons.
How can I make backface use alpha correctly?