CustomFader.cs

using UnityEngine;
using System.Collections;

public class CustomFader : MonoBehaviour {
  private Material screenMaterial;
  public bool startFaded = true;
  public float fadeSpeed = 0.5f;

  void Awake() {
    screenMaterial = GetComponent<Renderer>().material;
    SetAlphaTo(startFaded ? 1f : 0f);
  }

  private bool ActuallyFaded() {
    return screenMaterial.color.a == 1f;
  }

  private bool Fading() {
    return screenMaterial.color.a != 0f && screenMaterial.color.a != 1f;
  }

  public void ToggleFade() {
    if (!Fading()) {
      float target = ActuallyFaded() ? 0f : 1f;
      StartCoroutine(FadeTo(target));
    }
  }

  private IEnumerator FadeTo(float target) {
    float progress = 0f;
    float start = screenMaterial.color.a;
    while (screenMaterial.color.a != target) {
      yield return new WaitForEndOfFrame();
      float newAlpha = Mathf.Lerp(start, target, progress);
      SetAlphaTo(newAlpha);
      progress += fadeSpeed * Time.deltaTime;
    }
  }

  private void SetAlphaTo(float value) {
    Color newColor = screenMaterial.color;
    newColor.a = value;
    screenMaterial.color = newColor;
  }
}