FadeOutScreen.cs

using System.Collections;
using UnityEngine;

[RequireComponent(typeof(CanvasGroup))]
public class FadeOutScreen : MonoBehaviour{

    [SerializeField] private float fadeTime = 0.2f;
    [SerializeField] private AnimationCurve alphaCurve;

    private CanvasGroup fadeGroup;

    private void Start()
    {
        fadeGroup = GetComponent<CanvasGroup>();
    }

    public IEnumerator FadeOut()
    {
        fadeGroup.blocksRaycasts = true;

        float time = 0f;
        while(time < fadeTime)
        {
            fadeGroup.alpha = alphaCurve.Evaluate(time / fadeTime);

            time += Time.deltaTime;
            yield return null;
        }

        fadeGroup.alpha = alphaCurve.Evaluate(1f);
    }

    public IEnumerator FadeIn()
    {
        float time = fadeTime;
        while (time > 0f)
        {
            fadeGroup.alpha = alphaCurve.Evaluate(time / fadeTime);

            time -= Time.deltaTime;
            yield return null;
        }

        fadeGroup.alpha = alphaCurve.Evaluate(0f);

        fadeGroup.blocksRaycasts = false;
    }
}