Transparent sprite color change shader

Hello! I've got a transparent Sprite from a png sprite sheet, that is rendered by SpriteRenderer. I want to be able to switch it's fully transparent alpha with semi-transparent black color. Simply changing color doesn't work, since it doesn't change alpha color. I know there any many ways to do this - such as simply put another square sprite on top - or use two different versions of the same sprite, but I'm curios to do this using a shader. Here is what I've got so far:

 Shader "Custom/Darkened"
 {
 Properties
     {
     _MainTex("Base (RGB), Alpha (A)", 2D) = "black" {}
     }
     SubShader
     {
         LOD 200
         Tags
         {
             "Queue" = "Transparent"
             "IgnoreProjector" = "True"
             "RenderType" = "Transparent"
         }
         
         Pass
         {
             ColorMask RGBA
             Cull Off
             Lighting Off
             ZWrite Off
             Fog { Mode Off }
             Offset -1,-1
             Blend SrcAlpha OneMinusDstAlpha
             
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag            
             #include "UnityCG.cginc"
 
             sampler2D _MainTex;
             float4 _MainTex_ST;
     
             struct appdata_t
             {
                 float4 vertex : POSITION;
                 float2 texcoord : TEXCOORD0;
                 fixed4 color : COLOR;
             };
     
             struct v2f
             {
                 float4 vertex : SV_POSITION;
                 half2 texcoord : TEXCOORD0;
                 fixed4 color : COLOR;
             };
     
             v2f o;
 
             v2f vert (appdata_t v)
             {
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.texcoord = v.texcoord;
                 o.color = v.color
                 return o;
             }
                 
             fixed4 frag (v2f IN) : COLOR
             {
             IN.color.a=0.5;
             return tex2D(_MainTex, IN.texcoord) * IN.color;
             }
             ENDCG
         }
     }
 
 
 
     
 }
 

Alpha is removed, but it's completely black, not semi-transparent, I cannot see through it. What's the right way to do this?