Get real colors like white or black

I set Skybox solid color to Black but I got only grey.

When I set camera to render to texture then it seems I am getting real colors like black and white instead or lighter and darker grey tones.

I wish to understand what makes that effect.

Also I created shader that returns color only and assigned to box and still I can't make black box. Only grey box appear on screen.

 Shader "Unlit/Color"
 {
 Properties
     {
         _Color ("Color", Color) = (1,1,1,1)
     }
   SubShader {
         Pass {
             CGPROGRAM
                    #pragma vertex vert
             #pragma fragment frag
             
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
             };
 
             struct v2f
             {
                 float4 vertex : SV_POSITION;
             };
 
                  fixed4 _Color;
         
             v2f vert (appdata v)
             {
                 v2f o;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 return o;
             }
             
             fixed4 frag (v2f i) : SV_Target
             {
                 fixed4 col = _Color;
                 return col;
             }
             ENDCG
         }
     }
 }
 

I think if I assign _Color to be black then it should appear black on screen. But may be there some hidden rendering pass. Do I able to disable that pass or tune it? or at least read about it. How is it named?