CameraFader.cs

using UnityEngine;
using System.Collections;

public class CameraFader : MonoBehaviour
{
  public static CameraFader main;
  public Texture2D blackTexture;
  private float alpha = 0f;

  void Awake()
  {
    main = this;
  }

  void OnGUI()
  {
    GUI.color = new Color(0, 0, 0, alpha);
    GUI.DrawTexture(new Rect(0, 0, Screen.width, Screen.height ), blackTexture);
  }
  
  public void FadeIn(float duration)
  {
    StartCoroutine(FadeIn);
  }

  public void FadeOut(float duration)
  {
    StartCoroutine(FadeOut);
  }

  public IEnumerator FadeIn(float duration)
  {
    alpha = 1;

    float timePassed = 0;
    while(timePassed < duration)
      {
        timePassed += Time.deltaTime;
        alpha = Mathf.Lerp(1, 0, timePassed/duration);
        yield return null;
      }
    
    alpha = 0;
    yield return null;
  }

  public IEnumerator FadeOut(float duration)
  {
    alpha = 0;

    float timePassed = 0;
    while(timePassed < duration)
      {
        timePassed += Time.deltaTime;
        alpha = Mathf.Lerp(0, 1, timePassed/duration);
        yield return null;
      }
    
    alpha = 1;
    yield return null;
  }
  
}