Implementation of an CameraFadeScript for Unity 5 with Code only, no gameobjects or ui elements needed. (There has to be a main camera in the scene :P)

CamerFadeScript.cs

using System.Collections;
using UnityEngine;

// does need UnityStandardAssets > ImageEffects
// does not work if you use the ImageEffect "ScreenOverlay" on your camera for visual effects

// Usage:
// From a MonoBehaviour-Class: StartCoroutine(CameraFade.FadeOut(0.8f));
// From inside a coroutine (if you want to wait for finished fading): yield return CameraFade.FadeOut(0.5f);
// From inside a coroutine (if you dont want to wait): CameraFade.FadeOut(0.5f);

class CameraFadeScript : MonoBehaviour {
    
    [SerializeField] private Shader shader;
    private Material material;
    
    [Range(0, 1)] public float intensity;
    
    void Awake() {
        shader = shader ?? Shader.Find("Hidden/FadeToBlack");
        material = new Material(shader);
        material.SetColor("_Color", Color.black); 
    }

    void Update() {
        material = new Material(shader);
        material.SetFloat("_Intensity", intensity);
    }
    
    private void OnRenderImage(RenderTexture src, RenderTexture dest) {
        Graphics.Blit(src, dest, material);
    }
    
    public static CameraFadeScript SetupCameraFade() {
        var x = Camera.main.GetComponent<CameraFadeScript>();
        if (x == null) {
            x = Camera.main.gameObject.AddComponent<CameraFadeScript>();
        }
        return x;
    }

    public static IEnumerator FadeOut(float seconds) {
        var x = SetupCameraFade();
        x.intensity = 0; // fade out: 0 -> 1

        for(float i = 0; i < seconds; i+= Time.unscaledDeltaTime) {
            x.intensity = i / seconds;
            yield return null;
        }
        x.intensity = 1;
        yield return null;
    }

    public static IEnumerator FadeIn(float seconds) {
        var x = SetupCameraFade();
        x.intensity = 1; // fade in: 1 -> 0

        for (float i = 0; i < seconds; i += Time.unscaledDeltaTime) {
            x.intensity = 1 - i / seconds;
            yield return null;
        }
        x.intensity = 0;
        yield return null;
    }

}

FadeToBlack.shader

Shader "Hidden/FadeToBlack"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
		_Color("FadeColor", Color) = (0,0,0,1)
		_Intensity("Intensity", float) = 0
	}
	SubShader
	{
		// No culling or depth
		Cull Off ZWrite Off ZTest Always

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
			};

			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = v.uv;
				return o;
			}
			
			sampler2D _MainTex;
			float _Intensity;
			fixed4 _Color;

			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = tex2D(_MainTex, i.uv);
				
				col.rgb = col.rgb*(1-_Intensity) + (_Intensity * _Color) ;
				return col;
			}
			ENDCG
		}
	}
}

GameManager.cs

  public class GameManager : Singleton<GameManager> {

        void Awake() {
            var x = CameraFadeScript.SetupCameraFade();
            x.intensity = 1;
        }
        
        private IEnumerator Start() {
            var loadUIScene = SceneManager.LoadSceneAsync(3, LoadSceneMode.Additive);
            
            yield return null; // wait for first frame
            while (!loadUIScene.isDone) { yield return null; }
            yield return CameraFadeScript.FadeIn(1.4f);
        }